Gray-tone lithography for precise control of grating etch depth

ABSTRACT

Gray-tone lithography techniques for controlling the thickness profile of an overcoat layer on a surface-relief grating that has a non-uniform grating parameter (e.g., depth, duty cycle, or period), compensating for the non-uniform etch rate in a large area, defining etch/block regions, and/or controlling the thickness of the grating layer.

CROSS-REFERENCES TO RELATED APPLICATIONS

The following two U.S. patent applications listed below (which includethe present application) are being filed concurrently, and the entiredisclosure of the other application is hereby incorporated by referenceinto this application for all purposes:

-   -   application Ser. No. ______, filed Sep. 17, 2020, and entitled        “TECHNIQUES FOR MANUFACTURING VARIABLE ETCH DEPTH GRATINGS USING        GRAY-TONE LITHOGRAPHY” (Attorney Docket No.        FACTP124US/P200382US01); and    -   application Ser. No. ______, filed Sep. 17, 2020, and entitled        “GRAY-TONE LITHOGRAPHY FOR PRECISE CONTROL OF GRATING ETCH        DEPTH” (Attorney Docket No. FACTP125US/P200385US01).

BACKGROUND

An artificial reality system, such as a head-mounted display (HMD) orheads-up display (HUD) system, generally includes a near-eye display(e.g., in the form of a headset or a pair of glasses) configured topresent content to a user via an electronic or optic display in front ofthe user's eyes. The near-eye display may present virtual objects orcombine images of real objects with virtual objects, as in virtualreality (VR), augmented reality (AR), or mixed reality (MR)applications. For example, in an AR system, a user may view both imagesof virtual objects (e.g., computer-generated images (CGIs)) and thesurrounding environment by, for example, seeing through transparentdisplay glasses or lenses (often referred to as optical see-through).

One example of an optical see-through AR system may use awaveguide-based optical display, where the light of projected images maybe coupled into a waveguide (e.g., a transparent substrate), propagatewithin the waveguide, and then be coupled out of the waveguide atdifferent locations. In some optical see-through AR systems, the lightof the projected images may be coupled into and out of the waveguideusing diffractive optical elements, such as surface-relief gratings orholographic gratings. Light from the surrounding environment may alsopass through the diffractive optical elements in a see-through region ofthe waveguide and reach the user's eyes.

SUMMARY

This disclosure relates generally to surface-relief gratings. Morespecifically, disclosed herein are techniques for fabricatingsurface-relief gratings with variable depths and/or other gratingparameters (e.g., refractive indices). The surface-relief gratings withvariable depths and/or other grating parameters may be used to, forexample, reduce optical artifacts in the displayed images and/or thedisplay leakage in optical see-through waveguide displays for augmentedreality or mixed reality systems. Various inventive embodiments aredescribed herein, including devices, systems, methods, materials, andthe like.

According to certain embodiments, a method may include receiving asubstrate including a surface-relief grating formed thereon, depositingan overcoat layer having an uneven top surface on the surface-reliefgrating, depositing a gray-tone photoresist layer on the overcoat layer,exposing, through a gray-scale photomask, the gray-tone photoresistlayer to a light beam having a non-uniform intensity, removing exposedportions of the gray-tone photoresist layer, and etching the gray-tonephotoresist layer and the overcoat layer to form a flat top surface onthe overcoat layer.

In some embodiments, the surface-relief grating may be characterized byat least one of a variable etch depth, a variable grating period, avariable duty cycle, a variable thickness, or an uneven top surface. Insome embodiments, the gray-scale photomask may be characterized by atransmissivity profile corresponding to a height profile of a topsurface of the gray-tone photoresist layer. In some embodiments, an etchrate of the gray-tone photoresist layer may be between about 0.5 andabout 5 times of an etch rate of the overcoat layer during the etching.In some embodiments, the gray-tone photoresist layer may becharacterized by a non-binary response to exposure dosage such that adepth of an exposed portion of the gray-tone photoresist layer is afunction of the exposure dosage. The gray-tone photoresist layer may besensitive to light having a wavelength shorter than about 300 nm, 250nm, 193 nm, or 157 nm. In some embodiments, the light beam may includean ultraviolet light beam.

In some embodiments, the method may also include forming ananti-reflection coating layer or an angular selective transmission layeron the overcoat layer. In some embodiments, etching the gray-tonephotoresist layer and the overcoat layer may include etching thegray-tone photoresist layer and the overcoat layer using at least one ofan oxygen source including O₂, N₂O, CO₂, or CO; a nitrogen sourceincluding N₂, N₂O, or NH₃; or ions with an energy between about 100 andabout 500 eV.

According to certain embodiments, a method may include depositing agrating material layer on a substrate, forming a patterned hard mask ona first area of the grating material layer, depositing a photoresistmaterial layer that is sensitive to exposure light and has a non-binaryresponse to exposure dosage on the patterned hard mask and a second areaof the grating material layer, exposing the photoresist material layeron the patterned hard mask to a light beam having a non-uniform lightintensity for a period of time, developing the photoresist materiallayer to remove portions of the photoresist material layer exposed tothe light beam, etching the photoresist material layer and the gratingmaterial layer to form a grating with a variable depth in the gratingmaterial layer (where a portion of the photoresist material layerremains on the second area of the grating material layer), removing thepatterned hard mask, and etching the first area of the grating materiallayer and the portion of the photoresist material layer on the secondarea of the grating material layer.

In some embodiments of the method, an etch rate of the photoresistmaterial layer may be between about 0.5 and about 5 times of an etchrate of the grating material layer. In some embodiments, the light beammay include an ultraviolet light beam. In some embodiments, the methodmay further include exposing the photoresist material layer on thesecond area of the grating material layer to a light patterncorresponding to an alignment mark, where etching the first area of thegrating material layer and the photoresist material layer on the secondarea of the grating material layer may form the alignment mark in thesecond area of the grating material layer. In some embodiments, themethod may further include, after developing the photoresist materiallayer, curing the photoresist material layer using heat or ultravioletlight. In some embodiments, etching the photoresist material layer andthe grating material layer may include etching the photoresist materiallayer and the grating material layer using at least one of an oxygensource including O₂, N₂O, CO₂, or CO; a nitrogen source including N₂,N₂O, or NH₃; or ions with an energy between about 100 and about 500 eV.

According to certain embodiments, a method may include depositing agray-tone photoresist layer on a material layer to be etched by anetching system that is characterized by an uneven etch rate in an etcharea; exposing, through a gray-scale photomask, the gray-tonephotoresist layer to a light beam having a non-uniform intensity, wherea transmissivity of the gray-scale photomask is determined based on theuneven etch rate of the etching system; developing the gray-tonephotoresist layer to remove exposed portions of the gray-tonephotoresist layer, where remaining portions of the gray-tone photoresistlayer have an uneven top surface; and etching the remaining portions ofthe gray-tone photoresist layer and the material layer to be etched fora period of time.

In some embodiments of the method, an etch rate of the gray-tonephotoresist layer may be between about 0.5 and about 5 times of an etchrate of the material layer to be etched. In some embodiments, the lightbeam may include an ultraviolet beam. In some embodiments, the methodmay further include, after developing the gray-tone photoresist layer,curing the gray-tone photoresist layer. In some embodiments, thetransmissivity of the gray-scale photomask may be complementary to theuneven etch rate of the etching system.

In one embodiment, a waveguide display may include grating couplers thatmay diffractively couple display light into or out of a waveguide andrefractively transmit ambient light through the waveguide. Each of thegrating couplers may include two or more grating layers having differentrespective refractive indices and/or thickness profiles to reduce thecoupling of display light out of the waveguide display towards theambient environment.

This summary is neither intended to identify key or essential featuresof the claimed subject matter, nor is it intended to be used inisolation to determine the scope of the claimed subject matter. Thesubject matter should be understood by reference to appropriate portionsof the entire specification of this disclosure, any or all drawings, andeach claim. The foregoing, together with other features and examples,will be described in more detail below in the following specification,claims, and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments are described in detail below with reference tothe following figures.

FIG. 1 is a simplified block diagram of an example of an artificialreality system environment including a near-eye display according tocertain embodiments.

FIG. 2 is a perspective view of an example of a near-eye display in theform of a head-mounted display device for implementing some of theexamples disclosed herein.

FIG. 3 is a perspective view of an example of a near-eye display in theform of a pair of glasses for implementing some of the examplesdisclosed herein.

FIG. 4 illustrates an example of an optical see-through augmentedreality system including a waveguide display according to certainembodiments.

FIG. 5 illustrates propagations of display light and external light inan example of a waveguide display.

FIG. 6 illustrates an example of a slanted grating coupler in awaveguide display according to certain embodiments.

FIG. 7A illustrates an example of a waveguide-based near-eye displaywhere display light for all fields of view is substantially uniformlyoutput from different regions of a waveguide display.

FIG. 7B illustrates an example of a waveguide-based near-eye displaywhere display light may be coupled out of a waveguide display atdifferent angles in different regions of the waveguide display accordingto certain embodiments.

FIG. 8A illustrates a cross-section of an example of a slanted gratingwith variable etch depths according to certain embodiments.

FIG. 8B illustrates another cross-section of the example of the slantedgrating with variable etch depths shown in FIG. 8A according to certainembodiments.

FIG. 9 includes a flow chart illustrating an example of a process forfabricating a grating with a variable depth according to certainembodiments.

FIGS. 10A-10F illustrate an example of a process for manufacturing agrating with a variable grating depth according to certain embodiments.

FIGS. 11A-11C illustrate an example of a process for forming an etchmask having a desired thickness profile using a gray-scale photomaskaccording to certain embodiments.

FIGS. 12A-12D illustrate an example of a process for transferring thethickness profile of an etch mask to a underlying material layeraccording to certain embodiments.

FIG. 13 includes a flow chart illustrating an example of a process forfabricating a grating with a variable depth according to certainembodiments.

FIGS. 14A-14G illustrate an example of a process for manufacturing agrating with a variable depth according to certain embodiments.

FIG. 15A illustrates an example of a waveguide display that may generateoptical artifacts due to the diffraction of ambient light.

FIG. 15B illustrates an example of a waveguide display that may leakdisplay light into the ambient environment.

FIG. 16 illustrates examples of grating couplers in a waveguide displayaccording to certain embodiments.

FIG. 17 illustrates an example of a waveguide display including gratingcouplers with variable grating depths and variable refractive indicesaccording to certain embodiments.

FIGS. 18A-18F illustrate an example of a process for manufacturing agrating with a variable grating depth and an overcoat layer having aflat top according to certain embodiments.

FIG. 19A-19D illustrate an example of a method of controlling the heightprofile of a grating using gray-tone lithography according to certainembodiments.

FIG. 20 illustrates an example of a method of compensating fornon-uniform etch rates of an etching process using gray-tone lithographyaccording to certain embodiments.

FIG. 21 is a simplified block diagram of an example electronic system ofan example near-eye display for implementing some of the examplesdisclosed herein.

The figures depict embodiments of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative embodiments of the structuresand methods illustrated may be employed without departing from theprinciples, or benefits touted, of this disclosure.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION

Techniques disclosed herein relate generally to surface-relief gratingsfor optical systems, such as artificial reality systems. Morespecifically, disclosed herein are techniques for fabricatingsurface-relief gratings with desired depth profiles and other gratingparameters. The surface-relief gratings with the desired depth profilesand/or other grating parameters may be used to, for example, improveefficiencies, improve fields of view, reduce optical artifacts in thedisplayed images, and/or reduce the display leakage in opticalsee-through waveguide displays for augmented reality (AR) or mixedreality (MR) applications. Various inventive embodiments are describedherein, including devices, systems, methods, materials, processes, andthe like.

In an optical see-through waveguide display system, display light from alight source may be coupled into a waveguide using an input gratingcoupler, and may then be coupled out of the waveguide using outputgrating couplers for delivering to user's eyes. The waveguide and thegrating couplers may be transparent to visible light such that the usercan also view the ambient environment through the waveguide display. Inorder to improve power efficiency, image quality, security, and privacy,non-uniform grating couplers may be used. The non-uniform gratingcouplers may include, for example, surface-relief gratings that havevariable grating periods, etch depths, duty cycles, slant angles,refractive indies, and/or materials. Non-uniform grating couplers withvariable grating parameters may offer more degrees of freedom for tuningthe gratings to achieve the desired performance. However, it can be verychallenging to fabricate such non-uniform grating couplers, such asslanted surface-relief gratings with variable etch depths and/orvariable refractive indices.

According to certain embodiments, variable etch depth (VED) gratings maybe etched in a material layer (e.g., a film or a substrate) with auniform thickness using a gray-tone-last process. The gray-tone-lastprocess may include forming a patterned hard etch mask on the materiallayer to be etched, forming a gray-tone etch mask with a desire heightprofile (e.g., with a variable thickness) using gray-tone lithography,and then transferring the thickness profile of the gray-tone etch maskto the material layer by etching both the gray-tone etch mask and theunderlying material layer. The gray-tone etch mask may be formed from agray-tone photoresist layer by exposing the gray-tone photoresist layerto a non-uniform light beam using a gray-scale photomask and thenremoving the exposed portions of the gray-tone photoresist layer in adevelopment process. The gray-tone photoresist layer may have a linearor other non-binary response to the exposure dosage such that the depthof the exposed portion of the gray-tone photoresist layer in a regionmay be a function of the exposure dosage in the region. The gray-toneetch mask may have an etch rate similar or comparable to the etch rateof the material layer such that the thickness profile of the gray-toneetch mask may be transferred to the underlying material layer.

According to some embodiments, a gray-tone-first process may be usedalone or in combination with the gray-tone-last process to fabricategratings with variable grating parameters. The gray-tone-first processmay include performing a gray-tone lithography process to form a filmwith a desired non-uniform thickness profile on a substrate, forming ahard mask on the film with the non-uniform thickness profile, andetching the film using the hard mask to form a VED grating in the filmwith the non-uniform thickness profile. The hard mask may be formed bydepositing a hard mask material layer (e.g., a metal or metal alloymaterial, such as Cr) and a tri-layer mask on the film with thenon-uniform thickness profile. The tri-layer mask may be used to patternthe hard mask material layer and may include, for example, an organicdielectric layer at the bottom, an anti-reflection coating layer in themiddle, and a photoresist layer at the top. The photoresist layer may bepatterned and used as a mask for dry or wet etching to form a pattern inthe hard mask material layer. The patterned hard mask material layer maythen be used as the hard mask for etching the film.

In some embodiments, a bottom anti-reflection coating (BARC) layerand/or a top anti-reflection coating (TARC) layer may be used during thephotolithography to reduce light reflection and improve the resolutionand quality of the patterns.

According to certain embodiments, VED gratings including multiple layersof different materials having different refractive indices andnon-uniform thicknesses may be fabricated using techniques disclosedherein to improve the efficiencies, reduce certain optical artifacts,and/or undesired optical leakage that may cause interference, privacy,and/or security issues in a see-through waveguide display. For example,a waveguide display may include both surface-relief gratings made by thegray-tone-first process and surface-relief gratings made by thegray-one-last process. The input and output grating couplers may be madeusing different processes.

According to certain embodiments, gray-tone lithography may also be usedto control the thickness profile of an overcoat layer on asurface-relief grating that has a non-uniform grating parameter (e.g.,depth, duty cycle, or period), to compensate for the non-uniform etchrate in a large area, and/or to define etch/block regions or control thethickness of the grating layer.

For example, a slanted surface-relief grating coupler may include aslanted VED grating and an overcoat layer above the slanted VED grating.In some embodiments, the slanted surface-relief grating coupler may alsoinclude a selective transmission structure or anti-reflection structureabove the overcoat layer, for example, for optical artifact reduction.The slanted VED grating may be fabricated using the gray-tone-firstprocess and/or the gray-tone-last process disclosed herein. The overcoatlayer may be deposited on the slanted VED grating. Due to the variableetch depth of the slanted VED grating, the overcoat layer formed on theslanted VED gratings using existing techniques may have an uneven topsurface. For example, spin-on techniques may offer a relatively low-costand fast way of forming the overcoat layer above the slanted VEDgrating. But the top surface of the overcoat layer may not be evenbecause the spin-on material may follow the topography of the underlyingslanted VED grating, which may have varying slant angles, duty cycles,depths, and the like. Chemical-mechanical polishing (CM)) may be used toachieve a flat top surface on the grating, but may not precisely controlthe thickness of the overcoat layer on top of the slanted VED grating(referred to as overcoat burden).

According to some embodiments, an overcoat layer may be formed on agrating using, for example, the spin-on technique. A gray-tonephotoresist layer may be coated on the overcoat layer using, forexample, the spin-on technique. A gray-tone lithography process may thenbe performed using a gray-tone mask with the light transmissivitymirroring the overcoat layer topography to create a planar top surfaceon the gray-tone photoresist layer after the exposure and development.The gray-tone photoresist layer may have an etch rate similar orcomparable to an etch rate of the overcoat layer such that the gray-tonephotoresist layer and the underlying overcoat layer may be etched toleave a flat top surface on the overcoat layer. The etching rate andetch time may be controlled to control the thickness of the overcoatburden.

According to certain embodiments, gray-tone lithography may be used tocompensate for the non-uniform etch rate in a large area. A photoresistlayer with a non-uniform thickness may be formed on a film or substrateusing the gray-tone lithography and a gray-scale photomask having atransmissivity complementary to the non-uniform etch rate. Thephotoresist layer in an area with a higher etch rate may have a higherthickness, while the photoresist layer in an area with a lower etch ratemay have a lower thickness. The combination of the non-uniform thicknessof the photoresist layer and the non-uniform etch rate in the etch areamay resulted in a uniform effective etch rate of the a film orsubstrate.

According to certain embodiments, gray-tone lithography may be used todefine the etch/block region or control the thicknesses in differentregions of a grating. For example, a thick photoresist layer may beformed in regions where etching is not needed to block the regions frombeing etched.

In the following description, for the purposes of explanation, specificdetails are set forth in order to provide a thorough understanding ofexamples of the disclosure. However, it will be apparent that variousexamples may be practiced without these specific details. For example,devices, systems, structures, assemblies, methods, and other componentsmay be shown as components in block diagram form in order not to obscurethe examples in unnecessary detail. In other instances, well-knowndevices, processes, systems, structures, and techniques may be shownwithout necessary detail in order to avoid obscuring the examples. Thefigures and description are not intended to be restrictive. The termsand expressions that have been employed in this disclosure are used asterms of description and not of limitation, and there is no intention inthe use of such terms and expressions of excluding any equivalents ofthe features shown and described or portions thereof. The word “example”is used herein to mean “serving as an example, instance, orillustration.” Any embodiment or design described herein as “example” isnot necessarily to be construed as preferred or advantageous over otherembodiments or designs.

FIG. 1 is a simplified block diagram of an example of an artificialreality system environment 100 including a near-eye display 120 inaccordance with certain embodiments. Artificial reality systemenvironment 100 shown in FIG. 1 may include near-eye display 120, anoptional external imaging device 150, and an optional input/outputinterface 140, each of which may be coupled to an optional console 110.While FIG. 1 shows an example of artificial reality system environment100 including one near-eye display 120, one external imaging device 150,and one input/output interface 140, any number of these components maybe included in artificial reality system environment 100, or any of thecomponents may be omitted. For example, there may be multiple near-eyedisplays 120 monitored by one or more external imaging devices 150 incommunication with console 110. In some configurations, artificialreality system environment 100 may not include external imaging device150, optional input/output interface 140, and optional console 110. Inalternative configurations, different or additional components may beincluded in artificial reality system environment 100.

Near-eye display 120 may be a head-mounted display that presents contentto a user. Examples of content presented by near-eye display 120 includeone or more of images, videos, audio, or any combination thereof. Insome embodiments, audio may be presented via an external device (e.g.,speakers and/or headphones) that receives audio information fromnear-eye display 120, console 110, or both, and presents audio databased on the audio information. Near-eye display 120 may include one ormore rigid bodies, which may be rigidly or non-rigidly coupled to eachother. A rigid coupling between rigid bodies may cause the coupled rigidbodies to act as a single rigid entity. A non-rigid coupling betweenrigid bodies may allow the rigid bodies to move relative to each other.In various embodiments, near-eye display 120 may be implemented in anysuitable form-factor, including a pair of glasses. Some embodiments ofnear-eye display 120 are further described below with respect to FIGS. 2and 3. Additionally, in various embodiments, the functionality describedherein may be used in a headset that combines images of an environmentexternal to near-eye display 120 and artificial reality content (e.g.,computer-generated images). Therefore, near-eye display 120 may augmentimages of a physical, real-world environment external to near-eyedisplay 120 with generated content (e.g., images, video, sound, etc.) topresent an augmented reality to a user.

In various embodiments, near-eye display 120 may include one or more ofdisplay electronics 122, display optics 124, and an eye-tracking unit130. In some embodiments, near-eye display 120 may also include one ormore locators 126, one or more position sensors 128, and an inertialmeasurement unit (IMU) 132. Near-eye display 120 may omit any ofeye-tracking unit 130, locators 126, position sensors 128, and IMU 132,or include additional elements in various embodiments. Additionally, insome embodiments, near-eye display 120 may include elements combiningthe function of various elements described in conjunction with FIG. 1.

Display electronics 122 may display or facilitate the display of imagesto the user according to data received from, for example, console 110.In various embodiments, display electronics 122 may include one or moredisplay panels, such as a liquid crystal display (LCD), an organic lightemitting diode (OLED) display, an inorganic light emitting diode (ILED)display, a micro light emitting diode (μLED) display, an active-matrixOLED display (AMOLED), a transparent OLED display (TOLED), or some otherdisplay. For example, in one implementation of near-eye display 120,display electronics 122 may include a front TOLED panel, a rear displaypanel, and an optical component (e.g., an attenuator, polarizer, ordiffractive or spectral film) between the front and rear display panels.Display electronics 122 may include pixels to emit light of apredominant color such as red, green, blue, white, or yellow. In someimplementations, display electronics 122 may display a three-dimensional(3D) image through stereoscopic effects produced by two-dimensionalpanels to create a subjective perception of image depth. For example,display electronics 122 may include a left display and a right displaypositioned in front of a user's left eye and right eye, respectively.The left and right displays may present copies of an image shiftedhorizontally relative to each other to create a stereoscopic effect(i.e., a perception of image depth by a user viewing the image).

In certain embodiments, display optics 124 may display image contentoptically (e.g., using optical waveguides and couplers) or magnify imagelight received from display electronics 122, correct optical errorsassociated with the image light, and present the corrected image lightto a user of near-eye display 120. In various embodiments, displayoptics 124 may include one or more optical elements, such as, forexample, a substrate, optical waveguides, an aperture, a Fresnel lens, aconvex lens, a concave lens, a filter, input/output couplers, or anyother suitable optical elements that may affect image light emitted fromdisplay electronics 122. Display optics 124 may include a combination ofdifferent optical elements as well as mechanical couplings to maintainrelative spacing and orientation of the optical elements in thecombination. One or more optical elements in display optics 124 may havean optical coating, such as an anti-reflective coating, a reflectivecoating, a filtering coating, or a combination of different opticalcoatings.

Magnification of the image light by display optics 124 may allow displayelectronics 122 to be physically smaller, weigh less, and consume lesspower than larger displays. Additionally, magnification may increase afield of view of the displayed content. The amount of magnification ofimage light by display optics 124 may be changed by adjusting, adding,or removing optical elements from display optics 124. In someembodiments, display optics 124 may project displayed images to one ormore image planes that may be further away from the user's eyes thannear-eye display 120.

Display optics 124 may also be designed to correct one or more types ofoptical errors, such as two-dimensional optical errors,three-dimensional optical errors, or any combination thereof.Two-dimensional errors may include optical aberrations that occur in twodimensions. Example types of two-dimensional errors may include barreldistortion, pincushion distortion, longitudinal chromatic aberration,and transverse chromatic aberration. Three-dimensional errors mayinclude optical errors that occur in three dimensions. Example types ofthree-dimensional errors may include spherical aberration, comaticaberration, field curvature, and astigmatism.

Locators 126 may be objects located in specific positions on near-eyedisplay 120 relative to one another and relative to a reference point onnear-eye display 120. In some implementations, console 110 may identifylocators 126 in images captured by external imaging device 150 todetermine the artificial reality headset's position, orientation, orboth. A locator 126 may be a light emitting diode (LED), a corner cubereflector, a reflective marker, a type of light source that contrastswith an environment in which near-eye display 120 operates, or anycombination thereof. In embodiments where locators 126 are activecomponents (e.g., LEDs or other types of light emitting devices),locators 126 may emit light in the visible band (e.g., about 380 nm to750 nm), in the infrared (IR) band (e.g., about 750 nm to 1 mm), in theultraviolet band (e.g., about 10 nm to about 380 nm), in another portionof the electromagnetic spectrum, or in any combination of portions ofthe electromagnetic spectrum.

External imaging device 150 may include one or more cameras, one or morevideo cameras, any other device capable of capturing images includingone or more of locators 126, or any combination thereof. Additionally,external imaging device 150 may include one or more filters (e.g., toincrease signal to noise ratio). External imaging device 150 may beconfigured to detect light emitted or reflected from locators 126 in afield of view of external imaging device 150. In embodiments wherelocators 126 include passive elements (e.g., retroreflectors), externalimaging device 150 may include a light source that illuminates some orall of locators 126, which may retro-reflect the light to the lightsource in external imaging device 150. Slow calibration data may becommunicated from external imaging device 150 to console 110, andexternal imaging device 150 may receive one or more calibrationparameters from console 110 to adjust one or more imaging parameters(e.g., focal length, focus, frame rate, sensor temperature, shutterspeed, aperture, etc.).

Position sensors 128 may generate one or more measurement signals inresponse to motion of near-eye display 120. Examples of position sensors128 may include accelerometers, gyroscopes, magnetometers, othermotion-detecting or error-correcting sensors, or any combinationthereof. For example, in some embodiments, position sensors 128 mayinclude multiple accelerometers to measure translational motion (e.g.,forward/back, up/down, or left/right) and multiple gyroscopes to measurerotational motion (e.g., pitch, yaw, or roll). In some embodiments,various position sensors may be oriented orthogonally to each other.

IMU 132 may be an electronic device that generates fast calibration databased on measurement signals received from one or more of positionsensors 128. Position sensors 128 may be located external to IMU 132,internal to IMU 132, or any combination thereof. Based on the one ormore measurement signals from one or more position sensors 128, IMU 132may generate fast calibration data indicating an estimated position ofnear-eye display 120 relative to an initial position of near-eye display120. For example, IMU 132 may integrate measurement signals receivedfrom accelerometers over time to estimate a velocity vector andintegrate the velocity vector over time to determine an estimatedposition of a reference point on near-eye display 120. Alternatively,IMU 132 may provide the sampled measurement signals to console 110,which may determine the fast calibration data. While the reference pointmay generally be defined as a point in space, in various embodiments,the reference point may also be defined as a point within near-eyedisplay 120 (e.g., a center of IMU 132).

Eye-tracking unit 130 may include one or more eye-tracking systems. Eyetracking may refer to determining an eye's position, includingorientation and location of the eye, relative to near-eye display 120.An eye-tracking system may include an imaging system to image one ormore eyes and may optionally include a light emitter, which may generatelight that is directed to an eye such that light reflected by the eyemay be captured by the imaging system. For example, eye-tracking unit130 may include a non-coherent or coherent light source (e.g., a laserdiode) emitting light in the visible spectrum or infrared spectrum, anda camera capturing the light reflected by the user's eye. As anotherexample, eye-tracking unit 130 may capture reflected radio waves emittedby a miniature radar unit. Eye-tracking unit 130 may use low-power lightemitters that emit light at frequencies and intensities that would notinjure the eye or cause physical discomfort. Eye-tracking unit 130 maybe arranged to increase contrast in images of an eye captured byeye-tracking unit 130 while reducing the overall power consumed byeye-tracking unit 130 (e.g., reducing power consumed by a light emitterand an imaging system included in eye-tracking unit 130). For example,in some implementations, eye-tracking unit 130 may consume less than 100milliwatts of power.

Near-eye display 120 may use the orientation of the eye to, e.g.,determine an inter-pupillary distance (IPD) of the user, determine gazedirection, introduce depth cues (e.g., blur image outside of the user'smain line of sight), collect heuristics on the user interaction in theVR media (e.g., time spent on any particular subject, object, or frameas a function of exposed stimuli), some other functions that are basedin part on the orientation of at least one of the user's eyes, or anycombination thereof. Because the orientation may be determined for botheyes of the user, eye-tracking unit 130 may be able to determine wherethe user is looking. For example, determining a direction of a user'sgaze may include determining a point of convergence based on thedetermined orientations of the user's left and right eyes. A point ofconvergence may be the point where the two foveal axes of the user'seyes intersect. The direction of the user's gaze may be the direction ofa line passing through the point of convergence and the mid-pointbetween the pupils of the user's eyes.

Input/output interface 140 may be a device that allows a user to sendaction requests to console 110. An action request may be a request toperform a particular action. For example, an action request may be tostart or to end an application or to perform a particular action withinthe application. Input/output interface 140 may include one or moreinput devices. Example input devices may include a keyboard, a mouse, agame controller, a glove, a button, a touch screen, or any othersuitable device for receiving action requests and communicating thereceived action requests to console 110. An action request received bythe input/output interface 140 may be communicated to console 110, whichmay perform an action corresponding to the requested action. In someembodiments, input/output interface 140 may provide haptic feedback tothe user in accordance with instructions received from console 110. Forexample, input/output interface 140 may provide haptic feedback when anaction request is received, or when console 110 has performed arequested action and communicates instructions to input/output interface140. In some embodiments, external imaging device 150 may be used totrack input/output interface 140, such as tracking the location orposition of a controller (which may include, for example, an IR lightsource) or a hand of the user to determine the motion of the user. Insome embodiments, near-eye display 120 may include one or more imagingdevices to track input/output interface 140, such as tracking thelocation or position of a controller or a hand of the user to determinethe motion of the user.

Console 110 may provide content to near-eye display 120 for presentationto the user in accordance with information received from one or more ofexternal imaging device 150, near-eye display 120, and input/outputinterface 140. In the example shown in FIG. 1, console 110 may includean application store 112, a headset tracking module 114, an artificialreality engine 116, and an eye-tracking module 118. Some embodiments ofconsole 110 may include different or additional modules than thosedescribed in conjunction with FIG. 1. Functions further described belowmay be distributed among components of console 110 in a different mannerthan is described here.

In some embodiments, console 110 may include a processor and anon-transitory computer-readable storage medium storing instructionsexecutable by the processor. The processor may include multipleprocessing units executing instructions in parallel. The non-transitorycomputer-readable storage medium may be any memory, such as a hard diskdrive, a removable memory, or a solid-state drive (e.g., flash memory ordynamic random access memory (DRAM)). In various embodiments, themodules of console 110 described in conjunction with FIG. 1 may beencoded as instructions in the non-transitory computer-readable storagemedium that, when executed by the processor, cause the processor toperform the functions further described below.

Application store 112 may store one or more applications for executionby console 110. An application may include a group of instructions that,when executed by a processor, generates content for presentation to theuser. Content generated by an application may be in response to inputsreceived from the user via movement of the user's eyes or inputsreceived from the input/output interface 140. Examples of theapplications may include gaming applications, conferencing applications,video playback application, or other suitable applications.

Headset tracking module 114 may track movements of near-eye display 120using slow calibration information from external imaging device 150. Forexample, headset tracking module 114 may determine positions of areference point of near-eye display 120 using observed locators from theslow calibration information and a model of near-eye display 120.Headset tracking module 114 may also determine positions of a referencepoint of near-eye display 120 using position information from the fastcalibration information. Additionally, in some embodiments, headsettracking module 114 may use portions of the fast calibrationinformation, the slow calibration information, or any combinationthereof, to predict a future location of near-eye display 120. Headsettracking module 114 may provide the estimated or predicted futureposition of near-eye display 120 to artificial reality engine 116.

Artificial reality engine 116 may execute applications within artificialreality system environment 100 and receive position information ofnear-eye display 120, acceleration information of near-eye display 120,velocity information of near-eye display 120, predicted future positionsof near-eye display 120, or any combination thereof from headsettracking module 114. Artificial reality engine 116 may also receiveestimated eye position and orientation information from eye-trackingmodule 118. Based on the received information, artificial reality engine116 may determine content to provide to near-eye display 120 forpresentation to the user. For example, if the received informationindicates that the user has looked to the left, artificial realityengine 116 may generate content for near-eye display 120 that mirrorsthe user's eye movement in a virtual environment. Additionally,artificial reality engine 116 may perform an action within anapplication executing on console 110 in response to an action requestreceived from input/output interface 140, and provide feedback to theuser indicating that the action has been performed. The feedback may bevisual or audible feedback via near-eye display 120 or haptic feedbackvia input/output interface 140.

Eye-tracking module 118 may receive eye-tracking data from eye-trackingunit 130 and determine the position of the user's eye based on the eyetracking data. The position of the eye may include an eye's orientation,location, or both relative to near-eye display 120 or any elementthereof. Because the eye's axes of rotation change as a function of theeye's location in its socket, determining the eye's location in itssocket may allow eye-tracking module 118 to more accurately determinethe eye's orientation.

FIG. 2 is a perspective view of an example of a near-eye display in theform of an HMD device 200 for implementing some of the examplesdisclosed herein. HMD device 200 may be a part of, e.g., a VR system, anAR system, an MR system, or any combination thereof. HMD device 200 mayinclude a body 220 and a head strap 230. FIG. 2 shows a bottom side 223,a front side 225, and a left side 227 of body 220 in the perspectiveview. Head strap 230 may have an adjustable or extendible length. Theremay be a sufficient space between body 220 and head strap 230 of HMDdevice 200 for allowing a user to mount HMD device 200 onto the user'shead. In various embodiments, HMD device 200 may include additional,fewer, or different components. For example, in some embodiments, HMDdevice 200 may include eyeglass temples and temple tips as shown in, forexample, FIG. 3 below, rather than head strap 230.

HMD device 200 may present to a user media including virtual and/oraugmented views of a physical, real-world environment withcomputer-generated elements. Examples of the media presented by HMDdevice 200 may include images (e.g., two-dimensional (2D) orthree-dimensional (3D) images), videos (e.g., 2D or 3D videos), audio,or any combination thereof. The images and videos may be presented toeach eye of the user by one or more display assemblies (not shown inFIG. 2) enclosed in body 220 of HMD device 200. In various embodiments,the one or more display assemblies may include a single electronicdisplay panel or multiple electronic display panels (e.g., one displaypanel for each eye of the user). Examples of the electronic displaypanel(s) may include, for example, an LCD, an OLED display, an ILEDdisplay, a μLED display, an AMOLED, a TOLED, some other display, or anycombination thereof. HMD device 200 may include two eye box regions.

In some implementations, HMD device 200 may include various sensors (notshown), such as depth sensors, motion sensors, position sensors, and eyetracking sensors. Some of these sensors may use a structured lightpattern for sensing. In some implementations, HMD device 200 may includean input/output interface for communicating with a console. In someimplementations, HMD device 200 may include a virtual reality engine(not shown) that can execute applications within HMD device 200 andreceive depth information, position information, accelerationinformation, velocity information, predicted future positions, or anycombination thereof of HMD device 200 from the various sensors. In someimplementations, the information received by the virtual reality enginemay be used for producing a signal (e.g., display instructions) to theone or more display assemblies. In some implementations, HMD device 200may include locators (not shown, such as locators 126) located in fixedpositions on body 220 relative to one another and relative to areference point. Each of the locators may emit light that is detectableby an external imaging device.

FIG. 3 is a perspective view of an example of a near-eye display 300 inthe form of a pair of glasses for implementing some of the examplesdisclosed herein. Near-eye display 300 may be a specific implementationof near-eye display 120 of FIG. 1, and may be configured to operate as avirtual reality display, an augmented reality display, and/or a mixedreality display. Near-eye display 300 may include a frame 305 and adisplay 310. Display 310 may be configured to present content to a user.In some embodiments, display 310 may include display electronics and/ordisplay optics. For example, as described above with respect to near-eyedisplay 120 of FIG. 1, display 310 may include an LCD display panel, anLED display panel, or an optical display panel (e.g., a waveguidedisplay assembly).

Near-eye display 300 may further include various sensors 350 a, 350 b,350 c, 350 d, and 350 e on or within frame 305. In some embodiments,sensors 350 a-350 e may include one or more depth sensors, motionsensors, position sensors, inertial sensors, or ambient light sensors.In some embodiments, sensors 350 a-350 e may include one or more imagesensors configured to generate image data representing different fieldsof views in different directions. In some embodiments, sensors 350 a-350e may be used as input devices to control or influence the displayedcontent of near-eye display 300, and/or to provide an interactiveVR/AR/MR experience to a user of near-eye display 300. In someembodiments, sensors 350 a-350 e may also be used for stereoscopicimaging.

In some embodiments, near-eye display 300 may further include one ormore illuminators 330 to project light into the physical environment.The projected light may be associated with different frequency bands(e.g., visible light, infra-red light, ultra-violet light, etc.), andmay serve various purposes. For example, illuminator(s) 330 may projectlight in a dark environment (or in an environment with low intensity ofinfra-red light, ultra-violet light, etc.) to assist sensors 350 a-350 ein capturing images of different objects within the dark environment. Insome embodiments, illuminator(s) 330 may be used to project certainlight patterns onto the objects within the environment. In someembodiments, illuminator(s) 330 may be used as locators, such aslocators 126 described above with respect to FIG. 1.

In some embodiments, near-eye display 300 may also include ahigh-resolution camera 340. Camera 340 may capture images of thephysical environment in the field of view. The captured images may beprocessed, for example, by a virtual reality engine (e.g., artificialreality engine 116 of FIG. 1) to add virtual objects to the capturedimages or modify physical objects in the captured images, and theprocessed images may be displayed to the user by display 310 for AR orMR applications.

FIG. 4 illustrates an example of an optical see-through augmentedreality system 400 including a waveguide display according to certainembodiments. Augmented reality system 400 may include a projector 410and a combiner 415. Projector 410 may include a light source or imagesource 412 and projector optics 414. In some embodiments, light sourceor image source 412 may include one or more micro-LED devices describedabove. In some embodiments, image source 412 may include a plurality ofpixels that displays virtual objects, such as an LCD display panel or anLED display panel. In some embodiments, image source 412 may include alight source that generates coherent or partially coherent light. Forexample, image source 412 may include a laser diode, a vertical cavitysurface emitting laser, an LED, and/or a micro-LED described above. Insome embodiments, image source 412 may include a plurality of lightsources (e.g., an array of micro-LEDs described above), each emitting amonochromatic image light corresponding to a primary color (e.g., red,green, or blue). In some embodiments, image source 412 may include threetwo-dimensional arrays of micro-LEDs, where each two-dimensional arrayof micro-LEDs may include micro-LEDs configured to emit light of aprimary color (e.g., red, green, or blue). In some embodiments, imagesource 412 may include an optical pattern generator, such as a spatiallight modulator. Projector optics 414 may include one or more opticalcomponents that can condition the light from image source 412, such asexpanding, collimating, scanning, or projecting light from image source412 to combiner 415. The one or more optical components may include, forexample, one or more lenses, liquid lenses, mirrors, apertures, and/orgratings. For example, in some embodiments, image source 412 may includeone or more one-dimensional arrays or elongated two-dimensional arraysof micro-LEDs, and projector optics 414 may include one or moreone-dimensional scanners (e.g., micro-mirrors or prisms) configured toscan the one-dimensional arrays or elongated two-dimensional arrays ofmicro-LEDs to generate image frames. In some embodiments, projectoroptics 414 may include a liquid lens (e.g., a liquid crystal lens) witha plurality of electrodes that allows scanning of the light from imagesource 412.

Combiner 415 may include an input coupler 430 for coupling light fromprojector 410 into a substrate 420 of combiner 415. Combiner 415 maytransmit at least 50% of light in a first wavelength range and reflectat least 25% of light in a second wavelength range. For example, thefirst wavelength range may be visible light from about 400 nm to about650 nm, and the second wavelength range may be in the infrared band, forexample, from about 800 nm to about 1000 nm. Input coupler 430 mayinclude a volume holographic grating, a diffractive optical element(DOE) (e.g., a surface-relief grating), a slanted surface of substrate420, or a refractive coupler (e.g., a wedge or a prism). For example,input coupler 430 may include a reflective volume Bragg grating or atransmissive volume Bragg grating. Input coupler 430 may have a couplingefficiency of greater than 30%, 50%, 75%, 90%, or higher for visiblelight. Light coupled into substrate 420 may propagate within substrate420 through, for example, total internal reflection (TIR). Substrate 420may be in the form of a lens of a pair of eyeglasses. Substrate 420 mayhave a flat or a curved surface, and may include one or more types ofdielectric materials, such as glass, quartz, plastic, polymer,poly(methyl methacrylate) (PMMA), crystal, or ceramic. A thickness ofthe substrate may range from, for example, less than about 1 mm to about10 mm or more. Substrate 420 may be transparent to visible light.

Substrate 420 may include or may be coupled to a plurality of outputcouplers 440, each configured to extract at least a portion of the lightguided by and propagating within substrate 420 from substrate 420, anddirect extracted light 460 to an eyebox 495 where an eye 490 of the userof augmented reality system 400 may be located when augmented realitysystem 400 is in use. The plurality of output couplers 440 may replicatethe exit pupil to increase the size of eyebox 495 such that thedisplayed image is visible in a larger area. As input coupler 430,output couplers 440 may include grating couplers (e.g., volumeholographic gratings or surface-relief gratings), other diffractionoptical elements, prisms, etc. For example, output couplers 440 mayinclude reflective volume Bragg gratings or transmissive volume Bragggratings. Output couplers 440 may have different coupling (e.g.,diffraction) efficiencies at different locations. Substrate 420 may alsoallow light 450 from the environment in front of combiner 415 to passthrough with little or no loss. Output couplers 440 may also allow light450 to pass through with little loss. For example, in someimplementations, output couplers 440 may have a very low diffractionefficiency for light 450 such that light 450 may be refracted orotherwise pass through output couplers 440 with little loss, and thusmay have a higher intensity than extracted light 460. In someimplementations, output couplers 440 may have a high diffractionefficiency for light 450 and may diffract light 450 in certain desireddirections (i.e., diffraction angles) with little loss. As a result, theuser may be able to view combined images of the environment in front ofcombiner 415 and images of virtual objects projected by projector 410.

FIG. 5 illustrates propagations of display light 540 and external light530 in an example waveguide display 500 including a waveguide 510 and agrating coupler 520. Waveguide 510 may be a flat or curved transparentsubstrate with a refractive index n₂ greater than the free spacerefractive index n₁ (e.g., 1.0). Grating coupler 520 may be, forexample, a Bragg grating or a surface-relief grating.

Display light 540 may be coupled into waveguide 510 by, for example,input coupler 430 of FIG. 4 or other couplers (e.g., a prism or slantedsurface) described above. Display light 540 may propagate withinwaveguide 510 through, for example, total internal reflection. Whendisplay light 540 reaches grating coupler 520, display light 540 may bediffracted by grating coupler 520 into, for example, a 0^(th) orderdiffraction (i.e., reflection) light 542 and a −1st order diffractionlight 544. The 0^(th) order diffraction may propagate within waveguide510, and may be reflected by the bottom surface of waveguide 510 towardsgrating coupler 520 at a different location. The −1st order diffractionlight 544 may be coupled (e.g., refracted) out of waveguide 510 towardsthe user's eye, because a total internal reflection condition may not bemet at the bottom surface of waveguide 510 due to the diffraction angle.

External light 530 may also be diffracted by grating coupler 520 into,for example, a 0th order diffraction light 532 and a −1st orderdiffraction light 534. Both the 0^(th) order diffraction light 532 andthe −1st order diffraction light 534 may be refracted out of waveguide510 towards the user's eye. Thus, grating coupler 520 may act as aninput coupler for coupling external light 530 into waveguide 510, andmay also act as an output coupler for coupling display light 540 out ofwaveguide 510. As such, grating coupler 520 may act as a combiner forcombining external light 530 and display light 540. In general, thediffraction efficiency of grating coupler 520 (e.g., a surface-reliefgrating coupler) for external light 530 (i.e., transmissive diffraction)and the diffraction efficiency of grating coupler 520 for display light540 (i.e., reflective diffraction) may be similar or comparable.

In order to diffract light at a desired direction towards the user's eyeand to achieve a desired diffraction efficiency for certain diffractionorders, grating coupler 520 may include a blazed or slanted grating,such as a slanted Bragg grating or surface-relief grating, where thegrating ridges and grooves may be tilted relative to the surface normalof grating coupler 520 or waveguide 510.

FIG. 6 illustrates an example of a slanted grating 620 in a waveguidedisplay 600 according to certain embodiments. Slanted grating 620 may bean example of output couplers 440 or grating coupler 520. Waveguidedisplay 600 may include slanted grating 620 on a waveguide 610, such assubstrate 420 or waveguide 510. Slanted grating 620 may act as a gratingcoupler for couple light into or out of waveguide 610. In someembodiments, slanted grating 620 may include a periodic structure with aperiod p. For example, slanted grating 620 may include a plurality ofridges 622 and grooves 624 between ridges 622. Each period of slantedgrating 620 may include a ridge 622 and a groove 624, which may be anair gap or a region filled with a material with a refractive indexn_(g2). The ratio between the width w of a ridge 622 and the gratingperiod p may be referred to as duty cycle. Slanted grating 620 may havea duty cycle ranging, for example, from about 10% to about 90% orgreater. In some embodiments, the duty cycle may vary from period toperiod. In some embodiments, the period p of the slanted grating mayvary from one area to another on slanted grating 620, or may vary fromone period to another (i.e., chirped) on slanted grating 620.

Ridges 622 may be made of a material with a refractive index of n_(g1),such as silicon containing materials (e.g., SiO₂, Si₃N₄, SiC,SiO_(x)N_(y), or amorphous silicon), organic materials (e.g., spin oncarbon (SOC) or amorphous carbon layer (ACL) or diamond like carbon(DLC)), or inorganic metal oxide layers (e.g., TiO_(x), AlO_(x),TaO_(x), HfO_(x), etc.). Each ridge 622 may include a leading edge 630with a slant angel α and a trailing edge 640 with a slant angle β. Insome embodiments, leading edge 630 and training edge 640 of each ridge622 may be parallel to each other. In other words, slant angle α isapproximately equal to slant angle β. In some embodiments, slant angle αmay be different from slant angle β. In some embodiments, slant angle αmay be approximately equal to slant angle β. For example, the differencebetween slant angle α and slant angle β may be less than 20%, 10%, 5%,1%, or less. In some embodiments, slant angle α and slant angle β mayrange from, for example, about 30° or less to about 70% or larger.

In some implementations, grooves 624 between the ridges 622 may beover-coated or filled with a material having a refractive index n_(g2)higher or lower than the refractive index of the material of ridges 622.For example, in some embodiments, a high refractive index material, suchas Hafnia, Titania, Tantalum oxide, Tungsten oxide, Zirconium oxide,Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, and a highrefractive index polymer, may be used to fill grooves 624. In someembodiments, a low refractive index material, such as silicon oxide,alumina, porous silica, or fluorinated low index monomer (or polymer),may be used to fill grooves 624. As a result, the difference between therefractive index of the ridges and the refractive index of the groovesmay be greater than 0.1, 0.2, 0.3, 0.5, 1.0, or higher.

The user experience with an artificial reality system may depend onseveral optical characteristics of the artificial reality system, suchas the field of view (FOV), image quality (e.g., resolution), size ofthe eye box of the system (to accommodate for eye and/or head movement),distance of the eye relief, optical bandwidth, and brightness of thedisplayed image. In general, the FOV and the eye box need to be as largeas possible, the optical bandwidth needs to cover the visible band, andthe brightness of the displayed image needs to be high enough(especially for optical see-through AR systems).

In a waveguide-based near-eye display, the output area of the displaymay be much larger than the size of the eyebox of the near-eye displaysystem. The portion of light that may reach a user's eyes may depend onthe ratio between the size of the eyebox and the output area of thedisplay, which, in some cases, may be less than 10% for a certain eyerelief and field of view. In order to achieve a desired brightness ofthe displayed image perceived by user's eyes, the display light from theprojector or the light source may need to be increased significantly,which may increase the power consumption and cause some safety concerns.

FIG. 7A illustrates an example of a waveguide-based near-eye displaywhere display light for all fields of view is substantially uniformlyoutput from different regions of a waveguide display 710. The near-eyedisplay may include a projector 720 and waveguide display 710. Projector720 may be similar to projector 410 and may include a light source orimage source similar to light source or image source 412 and projectoroptics similar to projector optics 414. Waveguide display 710 mayinclude a waveguide (e.g., a substrate), one or more input couplers 712,and one or more output couplers 714. Input couplers 712 may beconfigured to couple display light from different fields of view (orviewing angles) into the waveguide. Output couplers 714 may beconfigured to couple display light out of the waveguide. The input andoutput couplers may include, for example, slanted surface-reliefgratings or volume Bragg gratings. In the example shown in FIG. 7A,output coupler 714 may have similar grating parameters across the fullregion of the output coupler other than parameters that may be varied toadjust the coupling efficiency to achieve more uniform output light.Thus, the display light may be partially coupled out of the waveguide atdifferent regions of waveguide display 710 in a similar manner as shownin FIG. 7A, where display light from all fields of view of the near-eyedisplay may be partially coupled out of the waveguide at any givenregion of waveguide display 710.

As also shown in FIG. 7A, the near-eye display system may have an eyeboxat a certain eyebox position 790 and having a limited size and thus alimited field of view 730. As such, not all light coupled out of thewaveguide in waveguide display 710 may reach the eyebox at eyeboxposition 790. For example, display light 732, 734, and 736 fromwaveguide display 710 may not reach the eyebox at eyebox position 790,and thus may not be received by the user's eyes, which may result insignificant loss of the optical power from projector 720.

In certain embodiments, an optical coupler (e.g., a slantedsurface-relief grating) for a waveguide-based display may include agrating coupler that includes multiple regions (or multiple multiplexedgrating). Different regions of the grating coupler may have differentangular selectivity characteristics (e.g., constructive interferenceconditions) for the incident display light such that, at any region ofthe waveguide-based display, diffraction light that would not eventuallyreach user's eyes may be suppressed (i.e., may not be diffracted by thegrating coupler so as to be coupled into or out of the waveguide andthus may continue to propagate within the waveguide), while light thatmay eventually reach the user's eyes may be diffracted by the gratingcoupler and be coupled into or out of the waveguide.

FIG. 7B illustrates an example of a waveguide-based near-eye displaywhere display light may be coupled out of a waveguide display 740 atdifferent angles in different regions of the waveguide display accordingto certain embodiments. Waveguide display 740 may include a waveguide(e.g., a substrate), one or more input couplers 742, and one or moreoutput couplers 744. Input couplers 742 may be configured to coupledisplay light from different fields of view (e.g., viewing angles) intothe waveguide, and output couplers 744 may be configured to coupledisplay light out of the waveguide. The input and output couplers mayinclude, for example, slanted surface-relief gratings or other gratings.The output couplers may have different grating parameters and thusdifferent angular selectivity characteristics at different regions ofthe output couplers. Thus, at each region of the output couplers, onlydisplay light that would propagate in a certain angular range towardsthe eyebox at eyebox position 790 of the near-eye display may be coupledout of the waveguide, while other display light may not meet the angularselectivity condition at the region and thus may not be coupled out ofthe waveguide. In some embodiments, the input couplers may also havedifferent grating parameters and thus different angular selectivitycharacteristics at different regions of the input couplers, and thus, ateach region of an input coupler, only display light from a respectivefield of view may be coupled into the waveguide. As a result, most ofthe display light coupled into the waveguide and propagating in thewaveguide can be efficiently sent to the eyebox, thus improving thepower efficiency of the waveguide-based near-eye display system.

The refractive index modulation of a slanted surface-relief grating, andother parameters of the slanted surface-relief grating, such as thegrating period, the slant angle, the duty cycle, the depth, and thelike, may be configured to selectively diffract incident light within acertain incident angular range and/or a certain wavelength band atcertain diffraction directions (e.g., within an angular range shown byfield of view 730). For example, when the refractive index modulation islarge (e.g., >0.2), a large angular bandwidth (e.g., >10°) may beachieved at the output couplers to provide a sufficiently large eyeboxfor the waveguide-based near-eye display system.

In many applications, to diffract light at a desired direction towardsthe user's eye, to achieve a desired diffraction efficiency for certaindiffraction orders, to increase the field of view and reduce rainbowartifacts of a waveguide display, a grating coupler may include a blazedor slanted grating, such as a slanted surface-relief grating, where thegrating ridges and grooves may be tilted relative to the surface normaldirection of the grating coupler or waveguide. In addition, in someembodiments, it may be desirable that the grating has a height or depthprofile that is non-uniform over the area of the grating, and/or has agrating period or duty cycle that varies across the grating, in order toimprove the performance of the grating, such as to achieve differentdiffraction characteristics (e.g., diffraction efficiencies and/ordiffraction angles) at different areas of the grating.

FIG. 8A illustrates a cross-section of an example of a slanted grating800 used in an example of a waveguide display according to certainembodiments. The cross-section shown in FIG. 8A may be in an x-z plane.Slanted grating 800 may include a grating region 820 in a substrate 810.Slanted grating 800 may act as a grating coupler for couple light intoor out of a waveguide. In some embodiments, slanted grating 800 mayinclude a structure with a period p, which may be a constant or may varyacross the area of slanted grating 800. Slanted grating 800 may includea plurality of ridges 822 and a plurality of grooves 824 between ridges822. Each period of slanted grating 800 may include a ridge 822 and agroove 824, which may be an air gap or a region filled with a materialwith a refractive index different from the refractive index of ridge822. The ratio between the width of a ridge 822 and the grating period pmay be referred to as the duty cycle. Slanted grating 800 may have aduty cycle ranging, for example, from about 30% to about 70%, or fromabout 10% to about 90% or greater. In some embodiments, the duty cyclemay vary from period to period or from area to area. In someembodiments, the period p of the slanted grating may vary from one areato another in slanted grating 800, or may vary from one period toanother (i.e., chirped) in slanted grating 800.

Ridges 822 may be made of a material, such as silicon containingmaterials (e.g., SiO₂, Si₃N₄, SiC, SiO_(x)N_(y), or amorphous silicon),organic materials (e.g., spin-on carbon, amorphous carbon layer, ordiamond like carbon), or inorganic metal oxide layers (e.g., TiO_(x),AlO_(x), TaO_(x), or HfO_(x)). Each ridge 822 may include a leading edge830 with a slant angel α and a trailing edge 840 with a slant angle β.In some embodiments, leading edge 830 and trailing edge 840 of eachridge 822 may be parallel to each other. In some embodiments, slantangle α may be different from slant angle β. In some embodiments, slantangle α may be approximately equal to slant angle β. For example, thedifference between slant angle α and slant angle β may be less than 20%,10%, 5%, 1%, or less. In some embodiments, slant angle α and slant angleβ may range from, for example, about 30° or less to about 70° or larger,such as about 45° or larger. In some embodiments, slant angle α and/orslant angle β may also vary from ridge to ridge in slanted grating 800.

Each groove 824 may have a depth d in the z direction, which may be aconstant or may vary across the area of slanted grating 800. In someembodiments, the depths of grooves 824 may vary across the area ofslanted grating 800 according to a pattern or a depth profile 850. Insome embodiments, the depths of grooves 824 may include multiple depthlevels, such as 8 depth levels, 16 depth levels, 32 depth levels, ormore. In some embodiments, the depths of grooves 824 may vary from 0 toabout 100 nm, 200 nm, 300 nm, or deeper. In some implementations,grooves 824 between ridges 822 may be over-coated or filled with amaterial having a refractive index higher or lower than the refractiveindex of the material of ridges 822. For example, in some embodiments, ahigh refractive index material, such as Hafnia, Titania, Tantalum oxide,Tungsten oxide, Zirconium oxide, Gallium sulfide, Gallium nitride,Gallium phosphide, silicon, or a high refractive index polymer, may beused to fill grooves 824. In some embodiments, a low refractive indexmaterial, such as silicon oxide, alumina, porous silica, or fluorinatedlow index monomer (or polymer), may be used to fill grooves 824. As aresult, the difference between the refractive index of the ridges andthe refractive index of the grooves may be greater than 0.1, 0.2, 0.3,0.5, 1.0, or higher.

FIG. 8B illustrates another cross-section of the example of slantedgrating 800 with variable etch depths shown in FIG. 8A according tocertain embodiments. The cross-section shown in FIG. 8B may be across-section along a line A-A shown in FIG. 8A, and thus may be in ay-z plane. A curve 860 in FIG. 8B illustrates the depth profile of aparticular grating groove 824, which may vary in the y direction. In theexample shown in FIG. 8B, grating region 820 may include aone-dimensional slanted grating with variable etch depths, where theone-dimensional slanted grating may include, in the x direction, theplurality of ridges 822 and the plurality of grooves 824.

In some embodiments, slanted grating 800 may include a two-dimensionalslanted grating with variable depths. The two-dimensional slantedgrating may include, in the x direction, the plurality of ridges 822 andthe plurality of grooves 824, and, in the y direction, a plurality ofridges and a plurality of grooves. The two-dimensional slanted gratingmay have a respective grating period in each of the x and y directions.In such embodiments, a cross-section of slanted grating 800 in a y-zplane may be similar to the cross-section of slanted grating 800 in anx-z plane as shown in FIG. 8A.

As such, slanted grating 800 may have a 3D structure, the physicaldimensions of which may vary in the x, y, and/or z directions. Forexample, the grating period or duty cycle of slanted grating 800 mayvary in the x-y plane and may also vary in the z-direction if slantangle α is different from slant angle β. The depths of grooves 824 inthe z direction may vary in the x and/or y directions. In someembodiments, the slant angle α and/or β with respect to the z directionmay also vary along the x and/or y directions in slanted grating 800.

The slanted surface-relief gratings with parameters and configurations(e.g., duty cycles, depths, or refractive index modulations) varyingover the regions of the gratings described above may be fabricated usingmany different nanofabrication techniques. The nanofabricationtechniques generally include a patterning process and a post-patterning(e.g., over-coating) process. The patterning process may be used to formslanted ridges or grooves of the slanted grating. There may be manydifferent nanofabrication techniques for forming the slanted ridges. Forexample, in some implementations, the slanted grating may be fabricatedusing lithography techniques including slanted etching. In someimplementations, the slanted grating may be fabricated using nanoimprintlithography (NIL) molding techniques, where a master mold includingslanted structures may be fabricated using, for example, slanted etchingtechniques, and may then be used to mold slanted gratings or differentgenerations of soft stamps for nanoimprinting. The post-patterningprocess may be used to over-coat the slanted ridges and/or to fill thegaps between the slanted ridges with a material having a differentrefractive index than the slanted ridges. The post-patterning processmay be independent from the patterning process. Thus, a samepost-patterning process may be used on slanted gratings fabricated usingany patterning technique.

Techniques and processes for fabricating slanted gratings describedherein are for illustration purposes only and are not intended to belimiting. A person skilled in the art would understand that variousmodifications may be made to the techniques described below. Forexample, in some implementations, some operations described below may beomitted. In some implementations, additional operations may be performedto fabricate the slanted gratings. Techniques disclosed herein may alsobe used to fabricate other slanted structures on various materials.

FIG. 9 is a flow chart 900 illustrating an example of a process forfabricating a grating with a variable depth profile according to certainembodiments. The process described in flow chart 900 may be referred toas a gray-tone-last process. The operations described in flow chart 900are for illustration purposes only and are not intended to be limiting.In various implementations, modifications may be made to flow chart 900to add additional operations, omit some operations, or change the orderof the operations. The operations described in flow chart 900 may beperformed using, for example, one or more semiconductor fabricationsystems, such as a spin coating system, a chemical vapor deposition(CVD) system, a physical vapor deposition (PVD) system, an ion or plasmaetching (e.g., ion beam etching (IBE), plasma etching (PE), or reactiveion etching (RIE)) system, a photolithography system, and the like.

At block 910, at least one material layer may be deposited on asubstrate. The substrate may be a transparent substrate, such as a glasssubstrate. The substrate may be flat or may be curved, and may include,for example, a lens, such as a vision correction lens or a lens forcorrecting one or more types of optical errors. The substrate mayinclude a material having a first refractive index, for example, fromabout 1.45 to about 2.4, such as about 1.9. The material layer mayinclude a uniform layer of a material having a second refractive index,such as close to the first refractive index. The material layer mayinclude, for example, a semiconductor material, a dielectric material, apolymer, and the like. In one example, the material layer may includeSiN, which may have a refractive index about 2.0. The material layer maybe deposited on the substrate by, for example, spin coating, PVD, CVD(e.g., low pressure chemical vapor deposition (LPCVD) or plasma-enhancedchemical vapor deposition (PECVD)), and the like. In some embodiments,multiple material layers with desired thicknesses and refractive indicesmay be sequentially deposited on the substrate. Each of the multiplematerial layers may be a material layer having a uniform thickness. Therefractive indices of the multiple material layers may graduallyincrease or gradually decrease.

At block 920, a hard mask layer may be formed on the at least onematerial layer. The hard mask layer may include, for example, a metal ormetal alloy material, such as chromium or chromium oxide. The hard masklayer may have a high resistance to dry etching, such as plasma etching.The hard mask layer may be formed on the at least one material layerusing, for example, PVD.

At block 930, the hard mask layer may be patterned to form a hard maskthat includes a desired light transmissivity pattern. In someembodiments, the hard mask layer may be patterned using a tri-layerstructure that includes an organic dielectric layer (ODL) at the bottom,an anti-reflection coating layer in the middle, and a photoresist layerat the top. The photoresist layer may be patterned and used as the etchmask to etch the anti-reflection coating layer, the ODL layer, and thehard mask layer to form the hard mask with the desired lighttransmissivity pattern. For example, the hard mask layer (e.g.,chromium) may be etched in an O₂ and Cl₂ or CCl₄ environment to form avolatile etching product CrO₂Cl₂.

At block 940, an etch mask layer may be deposited on the patterned hardmask layer. The etch mask layer may include a layer of a gray-tonephotoresist material that may have a linear or another known response toexposure dosage, such that the exposure depth may be a function of theexposure dosage. The gray-tone photoresist material may be deposited onthe hard mask layer by, for example, spin coating or spray coating.

At block 950, the etch mask layer may be exposed to a light beam througha gray-scale photomask that has different transmissivities at differentregions, and may then be developed to remove the exposed portions of thephotoresist material to form an etch mask with a variable thickness. Theetch mask layer may have a desired thickness profile, such as aramp-shaped profile or another profile that varies in one or twodimensions.

At block 960, the etch mask layer and the at least one material layermay be etched to linearly or nonlinearly transfer the thickness profileof the etch mask layer into the at least one material layer. The etchingmay be, for example, vertical or slanted dry etching using ion or plasmabeams. The etch time may be controlled to achieve the desired etchdepths in the at least one material layer. The etch mask layer with thevariable thickness may be completely etched by the etching process, ormay not be fully etched by the etching process but may be subsequentlyremoved by a photoresist stripping process using, for example, anorganic or inorganic stripper. Regions of the at least one materiallayer under the hard mask may not be etched, such that a grating with avariable depth and a pattern similar to the pattern of the hard mask maybe formed in the at least one material layer.

Optionally, at block 970, an overcoat layer with a desired refractiveindex may be formed on the etched grating to fill the grating grooves.For example, in some embodiments, a high refractive index material, suchas Hafnia, Titania, Tantalum oxide, Tungsten oxide, Zirconium oxide,Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, or a highrefractive index polymer, may be used to fill the grating grooves. Insome embodiments, a low refractive index material, such as siliconoxide, alumina, porous silica, or fluorinated low index monomer (orpolymer), may be used to fill the grating grooves.

FIGS. 10A-10F illustrate an example of a process 1000 for manufacturinga grating with a variable grating depth according to certainembodiments. The illustrated process may be an example of thegray-tone-last process described with respect to FIG. 9. FIG. 10A showsa substrate 1010 (e.g., a glass substrate) with a grating material layer1020 formed thereon. Even though one grating material layer 1020 isshown in the example, two or more grating material layers may bedeposited on substrate 1010. The two or more grating material layers mayhave different refractive indices and/or different thicknesses.

FIG. 10B shows mask layers formed on grating material layer 1010. Themask layers may include, for example, a hard mask layer 1030 (e.g., ametal or metal alloy material, such as Cr) and a tri-layer mask formedon hard mask layer 1030. The tri-layer mask may be used to pattern hardmask layer 1030. As described above, the tri-layer mask may include, forexample, an ODL 1040 at the bottom, a silicon-containing hard maskbottom (SHB) anti-reflection coating layer 1050 in the middle, and aphotoresist layer 1060 at the top. FIG. 10B shows that photoresist layer1060 has been patterned using, for example, a photolithography process.In some embodiments, a bottom anti-reflective coating (BARC) layer maybe formed on hard mask layer 1030 before forming the tri-layer mask.

FIG. 10C shows that an etching process is performed to remove parts ofthe tri-layer mask and parts of hard mask layer 1030 to form opening1042 in the mask layers, so as to form a pattern in hard mask layer1030. FIG. 10D shows that the tri-layer mask has been removed to exposethe patterned hard mask layer 1030. In some embodiments, a BARC layermay be formed on the patterned hard mask layer 1030 before the nextprocess step.

FIG. 10E shows that an etch mask 1070 is formed on the patterned hardmask layer 1030. Etch mask 1070 may have a desired height or thicknessprofile. Etch mask 1070 may be formed, using a gray-tone mask, in aphotoresist material layer that has a linear or other known response toexposure dosage. Because of the gray-tone mask, different regions of thephotoresist material layer may be exposed to different exposure dosesand thus the depths of the exposed photoresist material in differentregions may be different as well. Etch mask 1070 with the desiredthickness profile may be formed after the development of the photoresistmaterial to remove the exposed photoresist material.

FIG. 10F shows that a slanted etching process is performed using etchmask 1070 and patterned hard mask layer 1030 to transfer the pattern inpattern hard mask layer 1030 and the height profile of etch mask 1070into grating material layer 1020. Thus, a plurality of grating grooves1022 may be formed in grating material layer 1020. The etching processmay include a dry etching process, such as ion or plasma etching (e.g.,IBE, PE, or RIE). The ion or plasma beam may be slanted (e.g., at anangle greater than about 10°, 30°, or 45°) with respect to the surfacenormal direction of grating material layer 1020, such that gratinggrooves 1022 may be slanted to form a slanted grating in gratingmaterial layer 1020. After the etching, the remaining etch mask 1070 (ifany) and patterned hard mask layer 1030 may be removed, and the slantedgrating may optionally be coated with an overcoat layer as describedabove.

FIGS. 11A-11C illustrate an example of a process 1100 for forming anetch mask (e.g., etch mask 1070) having a desired thickness profileusing a gray-scale photomask according to certain embodiments. Theillustrated process 1100 may be an example of the process described withrespect to FIG. 10E. FIG. 11A shows a substrate 1110 (or a gratingmaterial layer formed on a substrate) having a hard mask 1120 (e.g., achromium-based hard mask) and a photoresist material layer 1130 formedthereon. Hard mask 1120 may be formed as described above with respect toFIGS. 10A-10D. Photoresist material layer 1130 may include a lowcontrast photoresist material that has a linear or other non-binaryresponse to exposure dosage. In some embodiments, the photoresistmaterial may be sensitive to light with a wavelength shorter than 300nm. In some embodiments, the photoresist material may be characterizedby an etch rate that is between about 0.5 and about 5 times of an etchrate of substrate 1110. In some embodiments, the photoresist materialmay be characterized by a linear response to ultraviolet (UV) light dosesuch that a depth of an exposed portion of the photoresist material is alinear function of the UV light dose. In some embodiments, thephotoresist material may include a positive-tone photoresist. In someembodiments, the photoresist material layer may include Poly(methylmethacrylate) (PMMA) sensitized with a photosensitive group. Thephotosensitive group may include, for example, at least one of anacyloximino group, methacrylonitrile, terpolymer of methyl methacrylate,oximino methacrylate, benzoic acids, N-acetylcarbazole, or indenone. Insome embodiments, the photoresist material layer may include at leastone of poly(methyl methacrylate)-r-poly(tert-butylmethacrylate)-r-poly(methyl methacrylate) and a photo acid generator,poly(methyl methacrylate)-r-poly(methacrylic acid),poly(α-methylstyrene-co-methyl chloroacrylate) and an acid generator,polycarbonate and a photo acid or base generator, polylactide and aphoto acid or base generator, or polyphthalaldehyde and a photo acidgenerator. In some embodiments, a BARC layer may be formed on hard mask1120 before depositing photoresist material layer 1130.

FIG. 11B shows that photoresist material layer 1130 is exposed to UVlight 1150 through a gray-scale photomask 1140. UV light 1150 may have awavelength shorter than, for example, 300 nm, such as between about 240nm and 280 nm, at 193 nm, at 157 nm, or lower (e.g., at a deep UVwavelength). Gray-scale photomask 1140 may include a transparentsubstrate and a layer having a UV light transmissivity varying acrossits area. As illustrated, in areas of photoresist material layer 1130corresponding to areas of gray-scale photomask 1140 that have highertransmissivities, the depths of the exposed portions 1132 of photoresistmaterial layer 1130 may be higher. FIG. 11C shows that a patternedphotoresist layer 1134 is formed in photoresist material layer 1130after the development and removal of exposed portions 1132.

FIGS. 12A-12D illustrate an example of a process for transferring thethickness profile of an etch mask to an underlying material layeraccording to certain embodiments. The illustrated process may be anexample of the process described above with respect to FIG. 10F. FIG.12A shows a patterned photoresist layer 1230 (e.g., patternedphotoresist layer 1134) on a substrate 1210 (e.g., substrate 1110, or agrating material layer formed on a substrate) having a hard mask 1220(e.g., hard mask 1120) formed thereon.

FIG. 12B shows an ion etching process that etches portions of patternedphotoresist layer 1230 and, in some areas, etches portions of substrate1210. The etch depth of substrate 1210 is the highest in areas ofsubstrate 1210 that correspond to areas of patterned photoresist layer1230 with the lowest thicknesses. In some embodiments, etching thepatterned photoresist layer 1230 and substrate 1210 may include etchingpatterned photoresist layer 1230 and substrate 1210 using at least oneof an oxygen source including O₂, N₂O, CO₂, or CO, a nitrogen sourceincluding N₂, N₂O, or NH₃, or ions with an energy between about 100-500eV. In some embodiments, the etching may be a slanted etching with aslant angle greater than, for example, about 10°, about 30°, or about45°.

FIG. 12C shows that patterned photoresist layer 1230 is completelyetched or the remaining portion of patterned photoresist layer 1230 hasbeen removed by a remover or a stripper (e.g., a solvent). Asillustrated in FIG. 12C, the depths of a grating groove 1212 insubstrate 1210 may be different in different areas of substrate 1210. Insome embodiments, the depths of grating grooves 1212 in substrate 1210may include at least 8 different depth levels. A maximum depth of thenon-uniform etch depth in substrate 1210 may be greater than about 100nm, greater than about 200 nm, greater than about 300 nm, or greaterthan about 500 nm. FIG. 12D shows that hard mask 1220 has been removedto expose the grating in substrate 1210.

Alternatively or additionally, a gray-tone-first process may be used tofabricate surface-relief gratings with variable etch depths. In thegray-tone-first process, gray-tone photolithography techniques may beused to form one or more grating material layers with desired thicknessprofiles, a patterned hard mask may be formed on the grating materiallayers, and the grating material layers may then be etched using thepatterned hard mask. The grating material layers may be etched throughto exposed the underlying substrate (which may be used an etch stopper).In some embodiments, the grating material layers formed using thegray-tone-first process may be etched using a gray-tone-last processdescribed above.

FIG. 13 includes a flow chart 1300 illustrating an example of a processfor fabricating a grating with a variable depth according to certainembodiments. The operations described in flow chart 1300 are forillustration purposes only and are not intended to be limiting. Invarious implementations, modifications may be made to flow chart 1300 toadd additional operations, to omit some operations, or to change theorder of the operations. The operations described in flow chart 1300 maybe performed by, for example, one or more semiconductor fabricationsystems, such as a spin coating system, a CVD system, a PVD system, anion or plasma etching (e.g., IBE, PE, or RIE) system, a photolithographysystem, and the like.

At block 1310, a grating material layer may be deposited on a substrateas described above with respect to, for example, block 910 or FIG. 10A.The substrate may be a transparent substrate, such as a glass substrate.The substrate may be flat or may be curved, and may include, forexample, a lens, such as a vision correction lens or a lens forcorrecting one or more types of optical errors. The substrate mayinclude a material having a first refractive index, for example, fromabout 1.45 to about 2.4, such as about 1.9. The grating material layermay include a uniform layer of a material having a second refractiveindex, such as close to the first refractive index. The grating materiallayer may include, for example, a semiconductor material, a dielectricmaterial, a polymer, and the like. The grating material layer may bedeposited on the substrate by, for example, spin coating, PVD, CVD(e.g., LPCVD or PECVD), and the like.

At block 1320, an etch mask layer with a variable thickness may beformed on the grating material layer. The etch mask layer may include adesired thickness profile, such as a ramp-shaped profile or anotherprofile that varies in one or two dimensions. As described above withrespect to blocks 940 and 950 and FIGS. 11A-11C, the etch mask layer maybe made by depositing a layer of gray-tone photoresist material that mayhave a linear or other non-binary response to exposure dosage, exposingthe layer of gray-tone photoresist material to light using a gray-scalephotomask that has different transmissivities at different regions, anddeveloping the layer of gray-tone photoresist material after exposure toremove the exposed portions of the photoresist material.

At block 1330, the etch mask layer and the grating material layer may beetched to change the thickness of the grating material layer by linearlyor nonlinearly transferring the thickness profile of the etch mask layerinto the grating material layer. The etching may be, for example,vertical dry etching using ion or plasma beams as described above. Theetch time may be controlled to achieve the desired thickness of theremaining grating material layer. The etch mask layer may be completelyetched by the etching process, or may not be fully etched by the etchingprocess and may be removed by a photoresist stripping process using, forexample, an organic or inorganic stripper.

The operations at block 1310 and/or blocks 1320-1330 may optionally berepeated to form additional grating material layers on the substrate.The additional grating material layers may each have a desirablethickness profile, such as a uniform thickness profile or a thicknessprofile that varies in one or two dimensions. The additional gratingmaterial layers may include different respective materials withdifferent respective refractive indices. Thus, the grating materiallayers may form a structure with a refractive index gradient. Forexample, the refractive index of the structure may gradually decrease(or increase) with the increase in the distance from the substrate. Insome embodiments, the grating material layers may have differentrespective thickness profiles such that the grating fabricated in thegrating material layers may reduce the leakage of the display light.

At block 1340, a patterned hard mask may be formed on the at least onegrating material layer. The hard mask may include, for example, a hardmask material layer (e.g., a metal or metal alloy material, such as Cr).As described above with respect to, for example, block 920 and FIGS.10B-10C, the hard mask material layer may be patterned using, forexample, a tri-layer mask that includes an ODL layer, a SHBanti-reflection coating layer, and a photoresist layer. The photoresistlayer may be patterned and used as the etch mask to etch the SHBanti-reflection coating layer, the ODL layer, and the hard mask materiallayer to form the hard mask with a desired light transmissivity pattern.

Optionally, at block 1350, an etch mask with a variable thickness may beformed on the hard mask as described above with respect to, for example,FIGS. 11A-11C and block 1320. The etch mask with the variable thicknessmay be formed by depositing a layer of gray-tone photoresist materialthat may have a linear or other non-binary response to exposure dosage,exposing the layer of gray-tone photoresist material to light using agray-scale photomask that has different transmissivities at differentregions, and developing the layer of gray-tone photoresist materialafter exposure to remove the exposed portions of the photoresistmaterial.

At block 1360, the at least one grating material layer may be etchedusing the hard mask (and the etch mask if present) to form a grating inthe at least one grating material layer. The etching may be vertical orslanted etching. For example, in some embodiments, the etching may beslanted etching using ion or plasma beams as described above. In someembodiments, the etch time may be controlled to achieve the desireddepth for the grating as shown in, for example, FIG. 10F and FIG. 12C.In some embodiments, the at least one grating material layer may have avariable overall thickness, the substrate or another layer may be usedas the etch stop layer for etching through the at least one gratingmaterial layer, and thus the etch time may not need to be preciselycontrolled.

Optionally, at block 1370, an overcoat layer with a desired refractiveindex may be formed on the etched grating to fill the grating grooves.For example, in some embodiments, a high refractive index material, suchas Hafnia, Titania, Tantalum oxide, Tungsten oxide, Zirconium oxide,Gallium sulfide, Gallium nitride, Gallium phosphide, silicon, or a highrefractive index polymer, may be used to fill the grating grooves. Insome embodiments, a lower refractive index material, such as siliconoxide, alumina, porous silica, or fluorinated low index monomer (orpolymer), may be used to fill the grating grooves.

FIGS. 14A-14G illustrate an example of a process 1400 for manufacturinga grating with a variable grating depth according to certainembodiments. The illustrated process 1400 may be an example of thegray-tone-first process described with respect to FIG. 13. FIG. 14Ashows a substrate 1410, which may be a transparent substrate, such as aglass substrate. Substrate 1410 may be flat or may be curved. Forexample, substrate 1410 may include a lens, such as a vision correctionlens or a lens for correcting one or more types of optical errors.Substrate 1410 may have a first refractive index, for example, fromabout 1.45 to about 2.4, such as about 1.9. A grating material layer1420 may be deposited on substrate 1410. Grating material layer 1420 mayinclude a uniform layer of a material having a second refractive index,such as close to the first refractive index. Grating material layer 1420may include, for example, a semiconductor material, a dielectricmaterial, a polymer, and the like. In one example, grating materiallayer 1420 may include SiN, which may have a refractive index about 2.0.Grating material layer 1420 may be deposited on substrate 1410 by, forexample, spin coating, PVD, CVD (e.g., LPCVD or PECVD), and the like.

FIG. 14B illustrates a gray-tone photoresist layer 1422 formed ongrating material layer 1420. Gray-tone photoresist layer 1422 mayinclude a desired thickness profile, such as a ramp-shaped profile or athickness profile that varies in one or two dimensions. As describedabove with respect to, for example, blocks 940, 950, and 1320 and FIGS.10E and 11A-11C, gray-tone photoresist layer 1422 may be made bydepositing a layer of gray-tone photoresist material, exposing the layerof gray-tone photoresist material to light using a gray-scale photomaskthat has different transmissivities at different regions, and developingthe layer of gray-tone photoresist material after exposure to remove theexposed portion of the gray-tone photoresist material.

FIG. 14C illustrates that grating material layer 1420 has been etchedusing gray-tone photoresist layer 1422 to linearly or nonlinearlytransfer the height profile of gray-tone photoresist layer 1422 intograting material layer 1420. The etching may be, for example, verticaldry etching using ion or plasma beams as described above. The etch timemay be controlled to achieve the desired thickness of grating materiallayer 1420. Gray-tone photoresist layer 1422 may be completely etched bythe etching process, or may not be fully etched by the etching processbut may be removed by a photoresist stripping process using, forexample, an organic or inorganic stripper.

FIG. 14D illustrates examples of mask layers formed on grating materiallayer 1420. The mask layers may include, for example, a hard maskmaterial layer 1430 (e.g., a metal or metal alloy material, such as Cr)and a tri-layer mask formed on the hard mask material layer. Asdescribed above with respect to, for example, FIGS. 10B and 10C, thetri-layer mask may be used to pattern hard mask material layer 1430 andmay include, for example, an organic dielectric layer 1432 at thebottom, an anti-reflection coating layer 1434 in the middle, and aphotoresist layer 1436 at the top. In some embodiments, a BARC layer maybe formed on hard mask material layer 1430 before forming the tri-layermask. FIG. 14D shows that photoresist layer 1436 has been patternedusing, for example, photolithography techniques.

FIG. 14E shows that a dry or wet etching process is performed to removeparts of the tri-layer mask and parts of hard mask material layer 1430to form opening 1440 in the mask layers, so as to form a pattern in hardmask material layer 1430. In the example shown in FIG. 14E, gratingmaterial layer 1420 may be used as the etch stop layer. FIG. 14F showsthat the tri-layer mask has been removed to expose the patterned hardmask material layer 1430.

FIG. 14G shows that a slanted etching process has been performed to etchgrating material layer 1420 using the patterned hard mask material layer1430, where substrate 1410 may be used as an etch stop layer. Thus,grating material layer 1420 may be etched down to substrate 1410 to forma plurality of grating grooves 1424. The etching process may include adry etching process, such as ion or plasma etching (e.g., IBE, PE, orRIE). The ion or plasma beam may be slanted with respect to the surfacenormal direction of substrate 1410 as described above, such that gratinggrooves 1424 may be slanted with respect to substrate 1410 to form aslanted grating in grating material layer 1420. Because of the variablethicknesses of grating material layer 1420, the grating formed ingrating material layer 1420 may be a grating with a variable depth.After the etching, patterned hard mask material layer 1430 may beremoved as shown in FIG. 14G. In some embodiments, the slanted gratingmay be coated with an overcoat layer (not shown in FIG. 14G) asdescribed above.

Grating couplers may not have diffraction efficiencies close to 100% andmay also diffract light in undesirable manners. Therefore, someartifacts may occur in the displayed images and/or some light may beleaked into the surrounding environment rather than reaching user's eye.For example, external light from a light source, such as the sun or alamp, may be undesirably diffracted by the grating couplers to causerainbow-colored images of the light source in the images displayed tothe user's eye. Display light may also be leaked into the environment tocause interference, security, and privacy concerns.

FIG. 15A illustrates the propagation of external light 1530 in anexample of a waveguide display 1500 with a grating coupler 1520 on thefront side of a waveguide 1510. External light 1530 may be diffracted bygrating coupler 1520 into a 0^(th) order diffraction light 1532 and a−1st order diffraction light 1534. The 0^(th) order diffraction light1532 may be refracted out of waveguide 1510 in a direction shown by alight ray 1536, or may be directed toward user's eyes. The −1st orderdiffraction light 1534 may be refracted out of waveguide 1510 in adirection shown by a light ray 1538, which may reach the eyebox anduser's eyes. For different wavelengths (colors), the 0^(th) orderdiffraction light may have a same diffraction angle, but the −1st orderdiffraction light may be wavelength dependent and thus may havedifferent diffraction angles for light of different colors to causerainbow-colored images.

FIG. 15B illustrates an example of leakage of display light in awaveguide display 1505. Waveguide display 1505 may be an example ofoptical see-through augmented reality system 400. Waveguide display 1505may include a substrate 1550, an input coupler 1560, and an outputcoupler 1570, which may be similar to substrate 420, input coupler 430,and output coupler 440, respectively. As illustrated, display light 1540may be coupled into substrate 1550 by input coupler 1560 such that thecoupled-in display light may propagate within substrate 1550 throughtotal internal reflection. As the display light reaches a surface ofsubstrate 1550 where output coupler 1570 is formed, a portion of thedisplay light may be reflectively diffracted such that the portion ofthe display light may be coupled out of substrate 1550 towards user'seyes as illustrated by a light beam 1580. A portion of the display lightentering output coupler 1570 may not be reflectively diffracted or maybe transmissively diffracted by output coupler 1570, and thus may becoupled out of the substrate towards the front of waveguide display 1505(e.g., in the z direction) as shown by a light beam 1590. Light beam1590 may be visible to viewers in front of waveguide display 1505. Thus,viewers in front of waveguide display 1505 may be able to view thedisplayed images, which may be undesirable in many circumstances.

According to certain embodiments, certain optical artifacts, such asrainbow images, may be reduced using, for example, slanted gratings. Theleakage of the display light may be reduced using, for example, agrating coupler characterized by a gradient refractive index orincluding multiple layers with different (e.g., increasing ordecreasing) refractive indices. In some embodiments, each of themultiple layers may have a respective thickness profile. The gratingcoupler with the gradient refractive index may also help to reducescattering artifacts and reflections at the interfaces between layers ofdifferent materials due to a smaller difference in the refractiveindices.

FIG. 16 illustrates an example of grating couplers with variable gratingdepths and variable refractive indices in a waveguide display 1600according to certain embodiments. The grating couplers may includemultiple grating layers, such as grating layers 1620, 1630, and 1640,formed on a substrate 1610 (e.g., a glass substrate). Grating layers1620, 1630, and 1640 may be formed using the gray-tone lithographytechniques described with respect to, for example, blocks 1310-1330 andFIGS. 14B and 14C. Even though three grating layers are shown in theexample, the multiple grating layers may include two or more layers. Asdescribed above, the multiple grating layers may be characterized bydifferent refractive indices. For example, in some embodiments, gratinglayer 1620 may have a refractive index greater than the refractive indexof grating layer 1630, which may be greater than the refractive index ofgrating layer 1640. In some embodiments, grating layer 1620 may have arefractive index greater than the refractive index of grating layer1630, while the refractive index of grating layer 1640 may be similar orequal to the refractive index of grating layer 1620. Each grating layerin grating layers 1620, 1630, and 1640 may have a non-uniform thicknessprofile, or may include regions with non-uniform thicknesses and regionswith a uniform thickness. Grating layers 1620, 1630, and 1640 may beformed using techniques described with respect to, for example, blocks1320 and 1330 and FIGS. 14B and 14C. The regions with non-uniformthicknesses and regions with a uniform thickness may be simultaneouslyformed in a same process or may be formed sequentially in differentprocesses.

In the example shown in FIG. 16, slanted grating couplers may be formedin different regions using different processes, such as thegray-tone-first technique, the gray-tone-last technique, or acombination of the gray-tone-first technique and the gray-tone-lasttechnique. For example, in a first region 1602, grating layers 1620,1630, and 1640 may each have non-uniform thicknesses, and a plurality ofgrating grooves 1650 may be etched in grating layers 1620, 1630, and1640 using substrate 1610 as the etch stop layer in a gray-tone-firstprocess. In a second region 1604, grating layers 1620, 1630, and 1640may each have a uniform thickness, and a plurality of grating grooves1652 may be etched in grating layers 1620, 1630, and 1640 using agray-tone etch mask that has a non-uniform thickness profile in agray-tone-last process. The etching may be slanted etching, such asslanted ion or plasma etching (e.g., IBE, PE, or RIE), such that gratinggrooves 1650 and 1652 may be slanted to form a slanted grating. Asdescribed above, in some embodiments, an overcoat layer with a desiredrefractive index may be formed on the grating couplers to fill gratinggrooves 1650 and 1652.

FIG. 17 illustrates an example of a waveguide display 1700 includinggrating couplers with variable grating depths and variable refractiveindices according to certain embodiments. Waveguide display 1700 mayinclude a substrate 1710, such as substrate 420, 1010, or 1410. In theillustrated example, waveguide display 1700 may include input and outputgrating couplers on both sides of substrate 1710. The input and outputgrating couplers may be etched in one or more grating material layersformed on substrate 1710, such as grating material layers 1720, 1730,and 1740 formed on the top side of substrate 1710 and grating materiallayers 1722, 1732, and 1742 formed on the bottom side of substrate 1710.The grating material layers may be formed using the gray-tonelithography techniques described with respect to, for example, blocks1310-1330 and FIGS. 14B and 14C. Even though three grating layers areshown on each side of substrate 1710 in the illustrated example, thegrating material layers may include one or more grating material layers,such as less than three grating material layers or more than threegrating material layers. The multiple grating material layers may becharacterized by different refractive indices. For example, the multiplegrating layers may have decreasing or increasing refractive indices. Asdescribed above with respect to FIG. 16, each grating material layer inthe grating material layers may have a non-uniform thickness profile, ormay have regions with non-uniform thicknesses and regions with a uniformthickness. For each grating material layer in the grating materiallayers, the regions with non-uniform thicknesses and regions with auniform thickness may be simultaneously formed in a same process or maybe formed sequentially in different processes.

As illustrated, waveguide display 1700 may include an input gratingcoupler 1780 and an output grating coupler 1790 on the top side ofsubstrate 1710, and an input grating coupler 1782 and an output gratingcoupler 1792 on the bottom side of substrate 1710. The grating couplersmay include vertical or slanted surface-relief gratings, with or withoutan overcoat layer. The grating couplers may have variable etch depths.In some embodiments, the grating couplers may also have variable gratingperiods, variable duty cycles, and/or variable slanted angles.

Input grating coupler 1780 and input grating coupler 1782 may be used tocouple display light into substrate 1710 as described above with respectto FIG. 4 and FIG. 15B. For example, input grating coupler 1780 may havea diffraction efficiency less than 100%, and the undiffracted displaylight may be diffracted (e.g., reflectively diffracted) by input gratingcoupler 1782. In some embodiments, input grating coupler 1780 and inputgrating coupler 1782 may be used to couple display light of differentcolors and/or from different fields of view into substrate 1710. In theexample shown in FIG. 17, input grating coupler 1780 may be formed in aregion where each grating material layer of grating material layers1720-1740 may have a different respective uniform thickness and adifferent respective material or composition (and thus a differentrespective refractive index). Input grating coupler 1780 may be formedusing the gray-tone-last process or a combination of the gray-tone-firstprocess and the gray-tone-last process. Input grating coupler 1782 maybe formed in a similar manner. Although not shown in FIG. 17, anovercoat layer may be formed on each of input grating coupler 1780 andinput grating coupler 1782.

Output grating coupler 1790 and output grating coupler 1792 may be usedto couple display light out of substrate 1710 and toward user's eyes asdescribed above with respect to, for example, FIG. 4 and FIG. 15B.Output grating coupler 1790 and output grating coupler 1792 may coupledifferent portions of the display light, such as different fractions ofthe total intensity, different color components, and/or light fordifferent fields of view, out of substrate 1710. In the example shown inFIG. 17, output grating coupler 1790 may be formed in a region whereeach grating material layer of grating material layers 1720-1740 mayhave a respective non-uniform thickness and a different respectivematerial or composition (and thus a different respective refractiveindex). Output grating coupler 1790 may be formed using thegray-tone-first process or a combination of the gray-tone-first processand the gray-tone-last process described above. For example, outputgrating coupler 1790 may be etched through grating material layers1720-1740, using substrate 1710 as the etch stop layer. Due to thenon-uniform thicknesses of grating material layers 1720-1740, outputgrating coupler 1790 may have a variable etch depth. Output gratingcoupler 1790 may alternatively be etched in grating material layers1720-1740 using a gray-tone photoresist layer that has a certainthickness profile as the etch mask for transferring the thicknessprofile into grating material layers 1720-1740. Output grating coupler1792 may be fabricated in a similar manner.

An overcoat layer 1750 and an overcoat layer 1752 may be formed onoutput grating coupler 1790 and output grating coupler 1792,respectively. As described above, the overcoat layer may include amaterial having a refractive index that is higher or lower than therefractive indices of grating material layers 1720-1740. A buffer layer1760 may be formed on overcoat layer 1750. A layer 1770 may be formed onbuffer layer 1760. Layer 1770 may be an anti-reflection coating layerthat may reduce the reflection of visible light at the top surface ofsubstrate 1710, including display light entering or exiting substrate1710 and ambient light for the see-through view. In some embodiments,layer 1770 may be an angular selective transmission layer, where ambientlight from grazing angles outside of the see-through field of view ofwaveguide display 1700 may be blocked such that it would not enter thegrating couplers to cause certain optical artifacts, such as rainbowimages described above. Layer 1770 may work for light in a broadwavelength range and a large angular range. In one example, layer 1770may include a grating with a very small grating period such that visiblelight diffracted by layer 1770 may have a large diffraction angle andthus may not reach user's eyes. Due to the small grating period, layer1770 may not result in see-through haze. A buffer layer 1762 and a layer1772 may be formed on overcoat layer 1752 and may be similar to bufferlayer 1760 and layer 1770, respectively.

As shown in, for example, FIG. 17, due to the non-uniform thicknesses ofgrating material layers 1720-1740, the top surface of overcoat layer1750 or 1752 may not be flat either because the coated material mayfollow the topography of the underlying output grating coupler 1790,which may have varying grating parameters, such as depths, slant angles,duty cycles, grating periods, and the like. For example, the top surfaceof overcoat layer 1750 at a region of output grating coupler 1790 havinga lower thickness may be lower than the top surface of overcoat layer1750 at a region of output grating coupler 1790 having a higherthickness. Due to the uneven top surface of overcoat layer 1750, it maybe difficult to manufacture other devices or components, such as layer1770, on overcoat layer 1750. Chemical-mechanical polishing techniquesmay be used to achieve a flat top surface on overcoat layer 1750, butmay not precisely control the thickness of overcoat layer 1750 on top ofthe slanted output grating coupler 1790.

According to certain embodiments, a gray-tone photoresist layer may becoated on the overcoat layer using, for example, the spin-on coatingtechnique. A gray-tone lithography process as described above may thenbe performed using a gray-scale photomask with the light transmissivitycorresponding to the overcoat layer topography to create a planar topsurface on the gray-tone photoresist layer after the exposure anddevelopment. The gray-tone photoresist layer may have an etch ratesimilar or comparable to an etch rate of the overcoat layer such thatthe gray-tone photoresist layer and the underlying overcoat layer may beetched in an etching process to leave a flat top on the overcoat layer.The etch rate and etch time may be controlled to control the thicknessof the overcoat burden.

FIGS. 18A-18F illustrate an example of a process for manufacturing agrating with an overcoat layer having a flat top according to certainembodiments. FIG. 18A shows a grating layer 1820 on a substrate 1810.Grating layer 1820 may include a surface-relief grating 1822 formedtherein, where surface-relief grating 1822 may have a variable etchdepth or a variable thickness. As described above, surface-reliefgrating 1822 may also have a variable grating period and/or a variableduty cycle.

FIG. 18B shows an overcoat layer 1830 coated on grating layer 1820.Overcoat layer 1830 may have an uneven top surface due to thenon-uniformity of the underlying surface-relief grating 1822. Forexample, in regions where the etch depth is higher, the top surface ofovercoat layer 1830 may be lower because the deeper grating grooves mayaccept more overcoat materials.

FIG. 18C shows a gray-tone photoresist layer 1840 coated on overcoatlayer 1830. As described above, gray-tone photoresist layer 1840 mayinclude a low contrast photoresist material that has a linear or othernon-binary response to exposure dosage. In some embodiments, thephotoresist material may be sensitive to light with a wavelength shorterthan about 300 nm. In some embodiments, the photoresist material may becharacterized by an etch rate that is between about 0.5 and about 5times of an etch rate of overcoat layer 1830. In some embodiments, thephotoresist material may be characterized by a linear response to UVlight dose such that a depth of an exposed portion of the photoresistmaterial is a linear function of the UV light dose. The photoresistmaterial may include a positive-tone photoresist material. In someembodiments, the photoresist material layer may include, for example,PMMA sensitized with a photosensitive group. The photosensitive groupmay include at least one of an acyloximino group, methacrylonitrile,terpolymer of methyl methacrylate, oximino methacrylate, benzoic acids,N-acetylcarbazole, or indenone. In some embodiments, the photoresistmaterial layer may include at least one of poly(methylmethacrylate)-r-poly(tert-butyl methacrylate)-r-poly(methylmethacrylate) and a photo acid generator, poly(methylmethacrylate)-r-poly(methacrylic acid), poly(α-methylstyrene-co-methylchloroacrylate) and an acid generator, polycarbonate and a photo acid orbase generator, polylactide and a photo acid or base generator, orpolyphthalaldehyde and a photo acid generator. Gray-tone photoresistlayer 1840 may be formed on overcoat layer 1830 by, for example, spincoating or spray coating. As illustrated in FIG. 18C, gray-tonephotoresist layer 1840 may have an uneven top surface due to the uneventop surface of the underlying overcoat layer 1830.

FIG. 18D shows a photolithography process, where gray-tone photoresistlayer 1840 may be exposed to a non-uniform light pattern 1860 for acertain period of time. The intensity of non-uniform light pattern 1860may correspond to the surface topology of gray-tone photoresist layer1840. For example, in regions where the top surface of gray-tonephotoresist layer 1840 is higher, the intensity of non-uniform lightpattern 1860 may be higher such that the exposed portion may have ahigher depth. In regions where the top surface of gray-tone photoresistlayer 1840 is lower, the intensity of non-uniform light pattern 1860 maybe lower such that the exposed portion may have a lower depth. As such,the interface between the exposed portion and the unexposed portion ofgray-tone photoresist layer 1840 may be approximately flat. Non-uniformlight pattern 1860 may be generated, for example, using a collimatedbeam with a uniform intensity and a gray-scale photomask 1850 that has atransmissivity corresponding to the desired intensity of non-uniformlight pattern 1860. In one embodiment, the topology of gray-tonephotoresist layer 1840 may be measured and the transmissivity ofgray-scale photomask 1850 may be determined based on the measuredtopology of gray-tone photoresist layer 1840.

FIG. 18E shows the gray-tone photoresist layer 1840 after exposure anddevelopment. As described above, due to the different exposure dosagesand thus different exposure depths at the different regions of gray-tonephotoresist layer 1840, the top surface of the unexposed portion 1842 ofgray-tone photoresist layer 1840 may be approximately flat. In someembodiments, the unexposed portion 1842 of gray-tone photoresist layer1840 may be cured (e.g., using UV light or heat) to desensitize thephotosensitive photoresist material.

FIG. 18F shows that a uniform etching process may be performed touniformly etch gray-tone photoresist layer 1840 and overcoat layer 1830.The parameters of the etching process, such as the etch rate and etchtime, can be set such that gray-tone photoresist layer 1840 may becompletely removed and the remaining portion 1832 of overcoat layer 1830may have a desired thickness. Due to the uniform etch rate, theresultant top surface of the remaining portion 1832 of overcoat layer1830 may be approximately flat. As such, it can be easier to fabricateother devices or components, such as an anti-reflection coating layer oran angular selective transmission layer described with respect to FIG.17, on the flat top surface of overcoat layer 1830.

FIGS. 19A-19D illustrate an example of a method of controlling theheight profile and grating region of a grating using gray-tonelithography according to certain embodiments. In some embodiments, itmay be desirable to prevent some regions of a grating layer from beingetched. For example, as shown in FIG. 17, certain regions of waveguidedisplay 1700 may not need to have grating structures. In someembodiments, it may be desirable to keep certain regions of the gratinglayer at a certain height different from surrounding regions, forexample, for use as an alignment mark or for improving the opticalmodulation transfer function (MTF) of the grating. The alignment markmay be used, for example, for mask alignment in subsequent processes orfor alignment during assembly. According to certain embodiments,gray-tone lithography may be used to define the etch regions and theblock regions, or to control the thicknesses in different regions of thegrating layer. For example, a thick photoresist layer may be formed inregions where etching is not needed to prevent the regions from beingetched.

FIG. 19A shows that a grating material layer 1920 is formed on asubstrate 1910. A hard mask 1930 may be formed on a region of gratingmaterial layer 1920 as described above with respect to, for example,FIGS. 10B-10D. FIG. 19B shows that a gray-tone photoresist layer 1940may be formed on hard mask 1930 and grating material layer 1920, and maybe patterned using the gray-tone photolithography process describedabove. In the illustrated example, a region 1942 of gray-tonephotoresist layer 1940 may have a uniform and high thickness, whereas aregion 1944 of gray-tone photoresist layer 1940 on hard mask 1930 mayhave a variable thickness. A slanted etching process may then beperformed to etch grating material layer 1920 using gray-tonephotoresist layer 1940 and hard mask 1930.

FIG. 19C shows that region 1944 of gray-tone photoresist layer 1940 hasbeen etched completely, while region 1942 of gray-tone photoresist layer1940 may have not been completely etched. A plurality of grating groovesmay be formed in grating material layer 1920. After the etching, hardmask 1930 may be stripped. FIG. 19D shows that region 1942 of gray-tonephotoresist layer 1940 and grating material layer 1920 may be furtheretched (e.g., vertically etched) to remove a region 1926 of gratingmaterial layer 1920, such that the top surface of a region 1924 ofgrating material layer 1920 may be higher than the top surface of otherregions of grating material layer 1920. In some embodiments, region 1942of gray-tone photoresist layer 1940 may have a non-uniform thickness toform a certain pattern, such as a registration feature (e.g., a cross)for alignment, in region 1924 of grating material layer 1920.

FIG. 20 illustrates an example of a method of compensating fornon-uniform etch rates of an etching process using gray-tone lithographyaccording to certain embodiments. In some etching systems, such as etchsystems that may have a large etch area (e.g., for wafer-level etching),the etch rates at different regions may be different. For example, theetch rate may be high at the center of the etch area, and may be low atthe edge of the etch area. In the illustrated example, a film 2020formed on a substrate 2010 may need to be etched. The etch rate of theetching system is shown by a pattern 2040, which shows that the etchrate may not be uniform across the entire etch area. The non-uniformetch rate may be measured and used to generate a gray-scale photomask.

To compensate for the non-uniform etch rate, a uniform layer of agray-tone photoresist layer 2030 may be formed on film 2020. Asdescribed above, gray-tone photoresist layer 2030 may have a linear oranother non-binary response to exposure dosage and may have an etch ratecomparable to the etch rate of film 2020. Gray-tone photoresist layer2030 may be exposed to a light beam with a uniform intensity through agray-scale photomask having a transmissivity complementary to themeasured etch rate of the etching system. After the exposure and thedevelopment processes, the remaining gray-tone photoresist layer 2030 inan area with a higher etch rate may have a higher thickness due to alower exposure dosage, while the remaining gray-tone photoresist layer2030 in an area with a lower etch rate may have a lower thickness due tothe higher exposure dosage.

Gray-tone photoresist layer 2030 with an uneven thickness and film 2020may then be etched using the etching system that has the etch rate shownby pattern 2040. A uniform etch depth as shown by a line 2022 in film2020 may be achieved after a certain etch period as a result of thecombination of the uneven thickness profile of gray-tone photoresistlayer 2030 and the uneven etch rates in different regions of the etcharea.

Embodiments of the invention may include or be implemented inconjunction with an artificial reality system. Artificial reality is aform of reality that has been adjusted in some manner beforepresentation to a user, which may include, for example, a virtualreality, an augmented reality, a mixed reality, a hybrid reality, orsome combination and/or derivatives thereof. Artificial reality contentmay include completely generated content or generated content combinedwith captured (e.g., real-world) content. The artificial reality contentmay include video, audio, haptic feedback, or some combination thereof,and any of which may be presented in a single channel or in multiplechannels (such as stereo video that produces a three-dimensional effectto the viewer). Additionally, in some embodiments, artificial realitymay also be associated with applications, products, accessories,services, or some combination thereof, that are used to, for example,create content in an artificial reality and/or are otherwise used in(e.g., perform activities in) an artificial reality. The artificialreality system that provides the artificial reality content may beimplemented on various platforms, including a head-mounted display (HMD)connected to a host computer system, a standalone HMD, a mobile deviceor computing system, or any other hardware platform capable of providingartificial reality content to one or more viewers.

FIG. 21 is a simplified block diagram of an example electronic system2100 of an example near-eye display (e.g., HMD device) for implementingsome of the examples disclosed herein. Electronic system 2100 may beused as the electronic system of an HMD device or other near-eyedisplays described above. In this example, electronic system 2100 mayinclude one or more processor(s) 2110 and a memory 2120. Processor(s)2110 may be configured to execute instructions for performing operationsat a number of components, and can be, for example, a general-purposeprocessor or microprocessor suitable for implementation within aportable electronic device. Processor(s) 2110 may be communicativelycoupled with a plurality of components within electronic system 2100. Torealize this communicative coupling, processor(s) 2110 may communicatewith the other illustrated components across a bus 2140. Bus 2140 may beany subsystem adapted to transfer data within electronic system 2100.Bus 2140 may include a plurality of computer buses and additionalcircuitry to transfer data.

Memory 2120 may be coupled to processor(s) 2110. In some embodiments,memory 2120 may offer both short-term and long-term storage and may bedivided into several units. Memory 2120 may be volatile, such as staticrandom access memory (SRAM) and/or DRAM and/or non-volatile, such asread-only memory (ROM), flash memory, and the like. Furthermore, memory2120 may include removable storage devices, such as secure digital (SD)cards. Memory 2120 may provide storage of computer-readableinstructions, data structures, program modules, and other data forelectronic system 2100. In some embodiments, memory 2120 may bedistributed into different hardware modules. A set of instructionsand/or code might be stored on memory 2120. The instructions might takethe form of executable code that may be executable by electronic system2100, and/or might take the form of source and/or installable code,which, upon compilation and/or installation on electronic system 2100(e.g., using any of a variety of generally available compilers,installation programs, compression/decompression utilities, etc.), maytake the form of executable code.

In some embodiments, memory 2120 may store a plurality of applicationmodules 2122 through 2124, which may include any number of applications.Examples of applications may include gaming applications, conferencingapplications, video playback applications, or other suitableapplications. The applications may include a depth sensing function oreye tracking function. Application modules 2122-2124 may includeparticular instructions to be executed by processor(s) 2110. In someembodiments, certain applications or parts of application modules2122-2124 may be executable by other hardware modules 2180. In certainembodiments, memory 2120 may additionally include secure memory, whichmay include additional security controls to prevent copying or otherunauthorized access to secure information.

In some embodiments, memory 2120 may include an operating system 2125loaded therein. Operating system 2125 may be operable to initiate theexecution of the instructions provided by application modules 2122-2124and/or manage other hardware modules 2180 as well as interfaces with awireless communication subsystem 2130 which may include one or morewireless transceivers. Operating system 2125 may be adapted to performother operations across the components of electronic system 2100including threading, resource management, data storage control and othersimilar functionality.

Wireless communication subsystem 2130 may include, for example, aninfrared communication device, a wireless communication device and/orchipset (such as a Bluetooth® device, an IEEE 802.11 device, a Wi-Fidevice, a WiMax device, cellular communication facilities, etc.), and/orsimilar communication interfaces. Electronic system 2100 may include oneor more antennas 2134 for wireless communication as part of wirelesscommunication subsystem 2130 or as a separate component coupled to anyportion of the system. Depending on desired functionality, wirelesscommunication subsystem 2130 may include separate transceivers tocommunicate with base transceiver stations and other wireless devicesand access points, which may include communicating with different datanetworks and/or network types, such as wireless wide-area networks(WWANs), wireless local area networks (WLANs), or wireless personal areanetworks (WPANs). A WWAN may be, for example, a WiMax (IEEE 802.16)network. A WLAN may be, for example, an IEEE 802.11x network. A WPAN maybe, for example, a Bluetooth network, an IEEE 802.15x, or some othertypes of network. The techniques described herein may also be used forany combination of WWAN, WLAN, and/or WPAN. Wireless communicationssubsystem 2130 may permit data to be exchanged with a network, othercomputer systems, and/or any other devices described herein. Wirelesscommunication subsystem 2130 may include a means for transmitting orreceiving data, such as identifiers of HMD devices, position data, ageographic map, a heat map, photos, or videos, using antenna(s) 2134 andwireless link(s) 2132. Wireless communication subsystem 2130,processor(s) 2110, and memory 2120 may together comprise at least a partof one or more of a means for performing some functions disclosedherein.

Embodiments of electronic system 2100 may also include one or moresensors 2190. Sensor(s) 2190 may include, for example, an image sensor,an accelerometer, a pressure sensor, a temperature sensor, a proximitysensor, a magnetometer, a gyroscope, an inertial sensor (e.g., a modulethat combines an accelerometer and a gyroscope), an ambient lightsensor, or any other similar module operable to provide sensory outputand/or receive sensory input, such as a depth sensor or a positionsensor. For example, in some implementations, sensor(s) 2190 may includeone or more inertial measurement units (IMUs) and/or one or moreposition sensors. An IMU may generate calibration data indicating anestimated position of the HMD device relative to an initial position ofthe HMD device, based on measurement signals received from one or moreof the position sensors. A position sensor may generate one or moremeasurement signals in response to motion of the HMD device. Examples ofthe position sensors may include, but are not limited to, one or moreaccelerometers, one or more gyroscopes, one or more magnetometers,another suitable type of sensor that detects motion, a type of sensorused for error correction of the IMU, or some combination thereof. Theposition sensors may be located external to the IMU, internal to theIMU, or some combination thereof. At least some sensors may use astructured light pattern for sensing.

Electronic system 2100 may include a display module 2160. Display module2160 may be a near-eye display, and may graphically present information,such as images, videos, and various instructions, from electronic system2100 to a user. Such information may be derived from one or moreapplication modules 2122-2124, virtual reality engine 2126, one or moreother hardware modules 2180, a combination thereof, or any othersuitable means for resolving graphical content for the user (e.g., byoperating system 2125). Display module 2160 may use liquid crystaldisplay (LCD) technology, LED technology (including, for example, OLED,ILED, μLED, AMOLED, TOLED, etc.), light emitting polymer display (LPD)technology, or some other display technology.

Electronic system 2100 may include a user input/output module 2170. Userinput/output module 2170 may allow a user to send action requests toelectronic system 2100. An action request may be a request to perform aparticular action. For example, an action request may be to start or endan application or to perform a particular action within the application.User input/output module 2170 may include one or more input devices.Example input devices may include a touchscreen, a touch pad,microphone(s), button(s), dial(s), switch(es), a keyboard, a mouse, agame controller, or any other suitable device for receiving actionrequests and communicating the received action requests to electronicsystem 2100. In some embodiments, user input/output module 2170 mayprovide haptic feedback to the user in accordance with instructionsreceived from electronic system 2100. For example, the haptic feedbackmay be provided when an action request is received or has beenperformed.

Electronic system 2100 may include a camera 2150 that may be used totake photos or videos of a user, for example, for tracking the user'seye position. Camera 2150 may also be used to take photos or videos ofthe environment, for example, for VR, AR, or MR applications. Camera2150 may include, for example, a complementary metal-oxide-semiconductor(CMOS) image sensor with a few millions or tens of millions of pixels.In some implementations, camera 2150 may include two or more camerasthat may be used to capture 3D images.

In some embodiments, electronic system 2100 may include a plurality ofother hardware modules 2180. Each of other hardware modules 2180 may bea physical module within electronic system 2100. While each of otherhardware modules 2180 may be permanently configured as a structure, someof other hardware modules 2180 may be temporarily configured to performspecific functions or temporarily activated. Examples of other hardwaremodules 2180 may include, for example, an audio output and/or inputmodule (e.g., a microphone or speaker), a near field communication (NFC)module, a rechargeable battery, a battery management system, awired/wireless battery charging system, etc. In some embodiments, one ormore functions of other hardware modules 2180 may be implemented insoftware.

In some embodiments, memory 2120 of electronic system 2100 may alsostore a virtual reality engine 2126. Virtual reality engine 2126 mayexecute applications within electronic system 2100 and receive positioninformation, acceleration information, velocity information, predictedfuture positions, or some combination thereof of the HMD device from thevarious sensors. In some embodiments, the information received byvirtual reality engine 2126 may be used for producing a signal (e.g.,display instructions) to display module 2160. For example, if thereceived information indicates that the user has looked to the left,virtual reality engine 2126 may generate content for the HMD device thatmirrors the user's movement in a virtual environment. Additionally,virtual reality engine 2126 may perform an action within an applicationin response to an action request received from user input/output module2170 and provide feedback to the user. The provided feedback may bevisual, audible, or haptic feedback. In some implementations,processor(s) 2110 may include one or more graphic processing units(GPUs) that may execute virtual reality engine 2126.

In various implementations, the above-described hardware and modules maybe implemented on a single device or on multiple devices that cancommunicate with one another using wired or wireless connections. Forexample, in some implementations, some components or modules, such asGPUs, virtual reality engine 2126, and applications (e.g., trackingapplication), may be implemented on a console separate from thehead-mounted display device. In some implementations, one console may beconnected to or support more than one HMD.

In alternative configurations, different and/or additional componentsmay be included in electronic system 2100. Similarly, functionality ofone or more of the components can be distributed among the components ina manner different from the manner described above. For example, in someembodiments, electronic system 2100 may be modified to include othersystem environments, such as an AR system environment and/or an MRenvironment.

The methods, systems, and devices discussed above are examples. Variousembodiments may omit, substitute, or add various procedures orcomponents as appropriate. For instance, in alternative configurations,the methods described may be performed in an order different from thatdescribed, and/or various stages may be added, omitted, and/or combined.Also, features described with respect to certain embodiments may becombined in various other embodiments. Different aspects and elements ofthe embodiments may be combined in a similar manner. Also, technologyevolves and, thus, many of the elements are examples that do not limitthe scope of the disclosure to those specific examples.

Specific details are given in the description to provide a thoroughunderstanding of the embodiments. However, embodiments may be practicedwithout these specific details. For example, well-known circuits,processes, systems, structures, and techniques have been shown withoutunnecessary detail in order to avoid obscuring the embodiments. Thisdescription provides example embodiments only, and is not intended tolimit the scope, applicability, or configuration of the invention.Rather, the preceding description of the embodiments will provide thoseskilled in the art with an enabling description for implementing variousembodiments. Various changes may be made in the function and arrangementof elements without departing from the spirit and scope of the presentdisclosure.

Also, some embodiments were described as processes depicted as flowdiagrams or block diagrams. Although each may describe the operations asa sequential process, many of the operations may be performed inparallel or concurrently. In addition, the order of the operations maybe rearranged. A process may have additional steps not included in thefigure. Furthermore, embodiments of the methods may be implemented byhardware, software, firmware, middleware, microcode, hardwaredescription languages, or any combination thereof. When implemented insoftware, firmware, middleware, or microcode, the program code or codesegments to perform the associated tasks may be stored in acomputer-readable medium such as a storage medium. Processors mayperform the associated tasks.

It will be apparent to those skilled in the art that substantialvariations may be made in accordance with specific requirements. Forexample, customized or special-purpose hardware might also be used,and/or particular elements might be implemented in hardware, software(including portable software, such as applets, etc.), or both. Further,connection to other computing devices such as network input/outputdevices may be employed.

With reference to the appended figures, components that can includememory can include non-transitory machine-readable media. The term“machine-readable medium” and “computer-readable medium,” as usedherein, refer to any storage medium that participates in providing datathat causes a machine to operate in a specific fashion. In embodimentsprovided hereinabove, various machine-readable media might be involvedin providing instructions/code to processing units and/or otherdevice(s) for execution. Additionally or alternatively, themachine-readable media might be used to store and/or carry suchinstructions/code. In many implementations, a computer-readable mediumis a physical and/or tangible storage medium. Such a medium may takemany forms, including, but not limited to, non-volatile media, volatilemedia, and transmission media. Common forms of computer-readable mediainclude, for example, magnetic and/or optical media such as compact disk(CD) or digital versatile disk (DVD), punch cards, paper tape, any otherphysical medium with patterns of holes, a RAM, a programmable read-onlymemory (PROM), an erasable programmable read-only memory (EPROM), aFLASH-EPROM, any other memory chip or cartridge, a carrier wave asdescribed hereinafter, or any other medium from which a computer canread instructions and/or code. A computer program product may includecode and/or machine-executable instructions that may represent aprocedure, a function, a subprogram, a program, a routine, anapplication (App), a subroutine, a module, a software package, a class,or any combination of instructions, data structures, or programstatements.

Those of skill in the art will appreciate that information and signalsused to communicate the messages described herein may be representedusing any of a variety of different technologies and techniques. Forexample, data, instructions, commands, information, signals, bits,symbols, and chips that may be referenced throughout the abovedescription may be represented by voltages, currents, electromagneticwaves, magnetic fields or particles, optical fields or particles, or anycombination thereof.

Terms, “and” and “or” as used herein, may include a variety of meaningsthat are also expected to depend at least in part upon the context inwhich such terms are used. Typically, “or” if used to associate a list,such as A, B, or C, is intended to mean A, B, and C, here used in theinclusive sense, as well as A, B, or C, here used in the exclusivesense. In addition, the term “one or more” as used herein may be used todescribe any feature, structure, or characteristic in the singular ormay be used to describe some combination of features, structures, orcharacteristics. However, it should be noted that this is merely anillustrative example and claimed subject matter is not limited to thisexample. Furthermore, the term “at least one of” if used to associate alist, such as A, B, or C, can be interpreted to mean any combination ofA, B, and/or C, such as A, AB, AC, BC, AA, ABC, AAB, AABBCCC, etc.

Further, while certain embodiments have been described using aparticular combination of hardware and software, it should be recognizedthat other combinations of hardware and software are also possible.Certain embodiments may be implemented only in hardware, or only insoftware, or using combinations thereof. In one example, software may beimplemented with a computer program product containing computer programcode or instructions executable by one or more processors for performingany or all of the steps, operations, or processes described in thisdisclosure, where the computer program may be stored on a non-transitorycomputer readable medium. The various processes described herein can beimplemented on the same processor or different processors in anycombination.

Where devices, systems, components or modules are described as beingconfigured to perform certain operations or functions, suchconfiguration can be accomplished, for example, by designing electroniccircuits to perform the operation, by programming programmableelectronic circuits (such as microprocessors) to perform the operationsuch as by executing computer instructions or code, or processors orcores programmed to execute code or instructions stored on anon-transitory memory medium, or any combination thereof. Processes cancommunicate using a variety of techniques, including, but not limitedto, conventional techniques for inter-process communications, anddifferent pairs of processes may use different techniques, or the samepair of processes may use different techniques at different times.

The specification and drawings are, accordingly, to be regarded in anillustrative rather than a restrictive sense. It will, however, beevident that additions, subtractions, deletions, and other modificationsand changes may be made thereunto without departing from the broaderspirit and scope as set forth in the claims. Thus, although specificembodiments have been described, these are not intended to be limiting.Various modifications and equivalents are within the scope of thefollowing claims.

What is claimed is:
 1. A method comprising: receiving a substrateincluding a surface-relief grating formed thereon; depositing anovercoat layer on the surface-relief grating, the overcoat layer havingan uneven top surface; depositing a gray-tone photoresist layer on theovercoat layer; exposing, through a gray-scale photomask, the gray-tonephotoresist layer to a light beam having a non-uniform intensity;removing exposed portions of the gray-tone photoresist layer; andetching the gray-tone photoresist layer and the overcoat layer to form aflat top surface on the overcoat layer.
 2. The method of claim 1,wherein the surface-relief grating is characterized by at least one of:a variable etch depth; a variable grating period; a variable duty cycle;a variable thickness; or an uneven top surface.
 3. The method of claim1, wherein the gray-scale photomask is characterized by a transmissivityprofile corresponding to a height profile of a top surface of thegray-tone photoresist layer.
 4. The method of claim 1, wherein an etchrate of the gray-tone photoresist layer is between 0.5 and 5 times of anetch rate of the overcoat layer during the etching.
 5. The method ofclaim 1, wherein the gray-tone photoresist layer is characterized by anon-binary response to exposure dosage such that a depth of an exposedportion of the gray-tone photoresist layer is a function of the exposuredosage.
 6. The method of claim 1, wherein the gray-tone photoresistlayer is sensitive to light having a wavelength shorter than 300 nm, 250nm, 193 nm, or 157 nm.
 7. The method of claim 1, wherein the light beamincludes an ultraviolet light beam.
 8. The method of claim 1, furthercomprising forming an anti-reflection coating layer or an angularselective transmission layer on the overcoat layer.
 9. The method ofclaim 1, wherein etching the gray-tone photoresist layer and theovercoat layer includes etching the gray-tone photoresist layer and theovercoat layer using at least one of: an oxygen source including O₂,N₂O, CO₂, or CO; a nitrogen source including N₂, N₂O, or NH₃; or ionswith an energy between 100 and 500 eV.
 10. A method comprising:depositing a grating material layer on a substrate; forming a patternedhard mask on a first area of the grating material layer; depositing aphotoresist material layer on the patterned hard mask and a second areaof the grating material layer, the photoresist material layer sensitiveto exposure light and having a non-binary response to exposure dosage;exposing the photoresist material layer on the patterned hard mask to alight beam for a period of time, the light beam characterized by anon-uniform light intensity; developing the photoresist material layerto remove portions of the photoresist material layer exposed to thelight beam; etching the photoresist material layer and the gratingmaterial layer to form a grating with a variable depth in the gratingmaterial layer, wherein a portion of the photoresist material layerremains on the second area of the grating material layer; removing thepatterned hard mask; and etching the first area of the grating materiallayer and the portion of the photoresist material layer on the secondarea of the grating material layer.
 11. The method of claim 10, whereinan etch rate of the photoresist material layer is between 0.5 and 5times of an etch rate of the grating material layer.
 12. The method ofclaim 10, wherein the light beam includes an ultraviolet light beam. 13.The method of claim 10, further comprising: exposing the photoresistmaterial layer on the second area of the grating material layer to alight pattern corresponding to an alignment mark, wherein etching thefirst area of the grating material layer and the photoresist materiallayer on the second area of the grating material layer forms thealignment mark in the second area of the grating material layer.
 14. Themethod of claim 10, further comprising, after developing the photoresistmaterial layer, curing the photoresist material layer using heat orultraviolet light.
 15. The method of claim 10, wherein etching thephotoresist material layer and the grating material layer includesetching the photoresist material layer and the grating material layerusing at least one of: an oxygen source including O₂, N₂O, CO₂, or CO; anitrogen source including N₂, N₂O, or NH₃; or ions with an energybetween 100 and 500 eV.
 16. A method comprising: depositing a gray-tonephotoresist layer on a material layer to be etched by an etching system,wherein the etching system is characterized by an uneven etch rate in anetch area; exposing, through a gray-scale photomask, the gray-tonephotoresist layer to a light beam having a non-uniform intensity,wherein a transmissivity of the gray-scale photomask is determined basedon the uneven etch rate of the etching system; developing the gray-tonephotoresist layer to remove exposed portions of the gray-tonephotoresist layer, wherein remaining portions of the gray-tonephotoresist layer have an uneven top surface; and etching the remainingportions of the gray-tone photoresist layer and the material layer to beetched for a period of time.
 17. The method of claim 16, wherein an etchrate of the gray-tone photoresist layer is between 0.5 and 5 times of anetch rate of the material layer to be etched.
 18. The method of claim16, wherein the light beam includes an ultraviolet beam.
 19. The methodof claim 16, further comprising, after developing the gray-tonephotoresist layer, curing the gray-tone photoresist layer.
 20. Themethod of claim 16, wherein the transmissivity of the gray-scalephotomask is complementary to the uneven etch rate of the etchingsystem.